goleetcode

This repository contains some of my solutions to problems that were provided by leetcode.com.

easy lvl

• 1. Two Sum (solved here, tests)

  Description

  Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target.

  You may assume that each input would have exactly one solution, and you may not use the same element twice.

  You can return the answer in any order.

• 21. Merge Two Sorted Lists (solved here, tests)

  Description

  You are given the heads of two sorted linked lists list1 and list2.

  Merge the two lists in a one sorted list. The list should be made by splicing together the nodes of the first two lists.

  Return the head of the merged linked list.

• 104. Maximum Depth of Binary Tree (solved here, tests)

  Description

  Given the root of a binary tree, return its maximum depth.

  A binary tree's maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

• 125. Valid Palindrome (solved here, tests)

  Description

  A phrase is a palindrome if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers.

  Given a string s, return true if it is a palindrome, or false otherwise.

• 169. Majority Element (solved here, tests)

  Description

  Given an array nums of size n, return the majority element.

  The majority element is the element that appears more than [n / 2] times. You may assume that the majority element always exists in the array.

• 191. Number of 1 Bits (solved here, tests)

  Description

  Write a function that takes an unsigned integer and returns the number of '1' bits it has (also known as the Hamming weight).

• 202. Happy Number (solved here, tests)

  Description

  Write an algorithm to determine if a number n is happy.

  A happy number is a number defined by the following process:

  Starting with any positive integer, replace the number by the sum of the squares of its digits.

  Repeat the process until the number equals 1 (where it will stay), or it loops endlessly in a cycle which does not include 1.

  Those numbers for which this process ends in 1 are happy.

  Return true if n is a happy number, and false if not.

• 217. Contains Duplicate (solved here, tests)

  Description

  Given an integer array nums, return true if any value appears at least twice in the array, and return false if every element is distinct.

• 232. Implement Queue using Stacks (solved here, tests)

  Description

  Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (push, peek, pop, and empty).

  Implement the MyQueue class:

  • void push(int x) Pushes element x to the back of the queue.

  • int pop() Removes the element from the front of the queue and returns it.

  • int peek() Returns the element at the front of the queue.

  • boolean empty() Returns true if the queue is empty, false otherwise.

  Notes:

  • You must use only standard operations of a stack, which means only push to top, peek/pop from top, size, and is empty operations are valid.

  • Depending on your language, the stack may not be supported natively. You may simulate a stack using a list or deque (double-ended queue) as long as you use only a stack's standard operations.

• 242. Valid Anagram (solved here, tests)

  Description

  Given two strings s and t, return true if t is an anagram of s, and false otherwise.

  An Anagram is a word or phrase formed by rearranging the letters of a different word or phrase, typically using all the original letters exactly once.

• 283. Move Zeroes (solved here, tests)

  Description

  Given an integer array nums, move all 0's to the end of it while maintaining the relative order of the non-zero elements.

  Note that you must do this in-place without making a copy of the array.

• 303. Range Sum Query - Immutable (solved here, tests)

  Description

  Given an integer array nums, handle multiple queries of the following type:

  1. Calculate the sum of the elements of nums between indices left and right inclusive where left <= right.

  Implement the NumArray class:

  • NumArray(int[] nums) Initializes the object with the integer array nums.

  • int sumRange(int left, int right) Returns the sum of the elements of nums between indices left and right inclusive (i.e. nums[left] + nums[left + 1] + ... + nums[right]).

• 344. Reverse String (solved here, tests)

  Description

  Write a function that reverses a string. The input string is given as an array of characters s.

  You must do this by modifying the input array in-place with O(1) extra memory.

• 389. Find the Difference (solved here, tests)

  Description

  You are given two strings s and t.

  String t is generated by random shuffling string s and then add one more letter at a random position.

  Return the letter that was added to t.

• 404. Sum of Left Leaves (solved here, tests)

  Description

  Given the root of a binary tree, return the sum of all left leaves.

  A leaf is a node with no children. A left leaf is a leaf that is the left child of another node.

• 496. Next Greater Element I (solved here, tests)

  Description

  The next greater element of some element x in an array is the first greater element that is to the right of x in the same array.

  You are given two distinct 0-indexed integer arrays nums1 and nums2, where nums1 is a subset of nums2. For each 0 <= i < nums1.length, find the index j such that nums1[i] == nums2[j] and determine the next greater element of nums2[j] in nums2. If there is no next greater element, then the answer for this query is -1.

  Return an array ans of length nums1.length such that ans[i] is the next greater element as described above.

• 566. Reshape the Matrix (solved here, tests)

  Description

  In MATLAB, there is a handy function called reshape which can reshape an m x n matrix into a new one with a different size r x c keeping its original data.

  You are given an m x n matrix mat and two integers r and c representing the number of rows and the number of columns of the wanted reshaped matrix.

  The reshaped matrix should be filled with all the elements of the original matrix in the same row-traversing order as they were.

  If the reshape operation with given parameters is possible and legal, output the new reshaped matrix; Otherwise, output the original matrix.

• 589. N-ary Tree Preorder Traversal (solved here, tests)

  Description

  Given the root of an n-ary tree, return the preorder traversal of its nodes' values.  Nary-Tree input serialization is represented in their level order traversal. Each group of children is separated by the null value.

• 709. To Lower Case (solved here, tests)

  Description

  Given a string s, return the string after replacing every uppercase letter with the same lowercase letter.

• 728. Self Dividing Numbers (solved here, tests)

  Description

  A self-dividing number is a number that is divisible by every digit it contains.

  • For example, 128 is a self-dividing number because 128 % 1 == 0, 128 % 2 == 0, and 128 % 8 == 0.

  A self-dividing number is not allowed to contain the digit zero.

  Given two integers left and right, return a list of all the self-dividing numbers in the range [left, right].

• 771. Jewels and Stones (solved here, tests)

  Description

  You're given strings jewels representing the types of stones that are jewels, and stones representing the stones you have. Each character in stones is a type of stone you have. You want to know how many of the stones you have are also jewels.

  Letters are case sensitive, so "a" is considered a different type of stone from "A".

• 804. Unique Morse Code Words (solved here, tests)

  Description

  International Morse Code defines a standard encoding where each letter is mapped to a series of dots and dashes, as follows:

  • 'a' maps to ".-",
  • 'b' maps to "-...",
  • 'c' maps to "-.-.", and so on.

  For convenience, the full table for the 26 letters of the English alphabet is given below:

  [".-","-...","-.-.","-..",".","..-.","--.","....","..",".---","-.-",".-..","--","-.","---",".--.","--.-",".-.","...","-","..-","...-",".--","-..-","-.--","--.."]

  Given an array of strings words where each word can be written as a concatenation of the Morse code of each letter.

  • For example, "cab" can be written as "-.-..--...", which is the concatenation of "-.-.", ".-", and "-...". We will call such a concatenation the transformation of a word.

  Return the number of different transformations among all words we have.

• 832. Flipping an Image (solved here, tests)

  Description

  Given an n x n binary matrix image, flip the image horizontally, then invert it, and return the resulting image.

  To flip an image horizontally means that each row of the image is reversed.

  • For example, flipping [1,1,0] horizontally results in [0,1,1].

  To invert an image means that each 0 is replaced by 1, and each 1 is replaced by 0.

  • For example, inverting [0,1,1] results in [1,0,0].

• 876. Middle of the Linked List (solved here, tests)

  Description

  Given the head of a singly linked list, return the middle node of the linked list.

  If there are two middle nodes, return the second middle node.

• 953. Verifying an Alien Dictionary (solved here, tests)

  Description

  In an alien language, surprisingly, they also use English lowercase letters, but possibly in a different order. The order of the alphabet is some permutation of lowercase letters.

  Given a sequence of words written in the alien language, and the order of the alphabet, return true if and only if the given words are sorted lexicographically in this alien language.

• 1021. Remove Outermost Parentheses (solved here, tests)

  Description

  A valid parentheses string is either empty "", "(" + A + ")", or A + B, where A and B are valid parentheses strings, and + represents string concatenation.

  For example, "", "()", "(())()", and "(()(()))" are all valid parentheses strings.

  A valid parentheses string s is primitive if it is nonempty, and there does not exist a way to split it into s = A + B, with A and B nonempty valid parentheses strings.

  Given a valid parentheses string s, consider its primitive decomposition: s = P1 + P2 + ... + Pk, where Pi are primitive valid parentheses strings.

  Return s after removing the outermost parentheses of every primitive string in the primitive decomposition of s.

• 1108. Defanging an IP Address (solved here, tests)

  Description

  Given a valid (IPv4) IP address, return a defanged version of that IP address.

  A defanged IP address replaces every period "." with "[.]".

• 1221. Split a String in Balanced Strings (solved here, tests)

  Description

  Balanced strings are those that have an equal quantity of 'L' and 'R' characters.

  Given a balanced string s, split it in the maximum amount of balanced strings.

  Return the maximum amount of split balanced strings.

• 1232. Check If It Is a Straight Line (solved here, tests)

  Description

  You are given an array coordinates, coordinates[i] = [x, y], where [x, y] represents the coordinate of a point. Check if these points make a straight line in the XY plane.

• 1281. Subtract the Product and Sum of Digits of an Integer (solved here, tests)

  Description

  Given an integer number n, return the difference between the product of its digits and the sum of its digits.

• 1290. Convert Binary Number in a Linked List to Integer (solved here, tests)

  Description

  Given head which is a reference node to a singly-linked list. The value of each node in the linked list is either 0 or 1. The linked list holds the binary representation of a number.

  Return the decimal value of the number in the linked list.

• 1313. Decompress Run-Length Encoded List (solved here, tests)

  Description

  We are given a list nums of integers representing a list compressed with run-length encoding.

  Consider each adjacent pair of elements [freq, val] = [nums[2*i], nums[2*i+1]] (with i >= 0). For each such pair, there are freq elements with value val concatenated in a sublist. Concatenate all the sublists from left to right to generate the decompressed list.

  Return the decompressed list.

• 1342. Number of Steps to Reduce a Number to Zero (solved here, tests)

  Description

  Given an integer num, return the number of steps to reduce it to zero.

  In one step, if the current number is even, you have to divide it by 2, otherwise, you have to subtract 1 from it.

• 1356. Sort Integers by The Number of 1 Bits (solved here, tests)

  Description

  You are given an integer array arr. Sort the integers in the array in ascending order by the number of 1's in their binary representation and in case of two or more integers have the same number of 1's you have to sort them in ascending order. Return the array after sorting it.

• 1365. How Many Numbers Are Smaller Than the Current Number (solved here, tests)

  Description

  Given the array nums, for each nums[i] find out how many numbers in the array are smaller than it. That is, for each nums[i] you have to count the number of valid j's such that j != i and nums[j] < nums[i].

  Return the answer in an array.

• 1389. Create Target Array in the Given Order (solved here, tests)

  Description

  Given two arrays of integers nums and index. Your task is to create target array under the following rules:

  -Initially target array is empty.

  -From left to right read nums[i] and index[i], insert at index index[i] the value nums[i] in target array.

  -Repeat the previous step until there are no elements to read in nums and index.

  -Return the target array.

  It is guaranteed that the insertion operations will be valid.

• 1431. Kids With the Greatest Number of Candies (solved here, tests)

  Description

  There are n kids with candies. You are given an integer array candies, where each candies[i] represents the number of candies the i-th kid has, and an integer extraCandies, denoting the number of extra candies that you have.

  Return a boolean array result of length n, where result[i] is true if, after giving the i-th kid all the extraCandies, they will have the greatest number of candies among all the kids, or false otherwise.

  Note that multiple kids can have the greatest number of candies.

• 1436. Destination City (solved here, tests)

  Description

  You are given the array paths, where paths[i] = [cityAi, cityBi] means there exists a direct path going from cityAi to cityBi. Return the destination city, that is, the city without any path outgoing to another city.

  It is guaranteed that the graph of paths forms a line without any loop, therefore, there will be exactly one destination city.

• 1470. Shuffle the Array (solved here, tests)

  Description

  Given the array nums consisting of 2n elements in the form [x1,x2,...,xn,y1,y2,...,yn].

  Return the array in the form [x1,y1,x2,y2,...,xn,yn].

• 1480. Running Sum of 1d Array (solved here, tests)

  Description

  Given an array nums. We define a running sum of an array as runningSum[i] = sum(nums[0]…nums[i]).

  Return the running sum of nums.

• 1486. XOR Operation in an Array (solved here, tests)

  Description

  You are given an integer n and an integer start.

  Define an array nums where nums[i] = start + 2 * i (0-indexed) and n == nums.length.

  Return the bitwise XOR of all elements of nums.

• 1502. Can Make Arithmetic Progression From Sequence (solved here, tests)

  Description

  A sequence of numbers is called an arithmetic progression if the difference between any two consecutive elements is the same.

  Given an array of numbers arr, return true if the array can be rearranged to form an arithmetic progression. Otherwise, return false.

• 1512. Number of Good Pairs (solved here, tests)

  Description

  Given an array of integers nums, return the number of good pairs.

  A pair (i, j) is called good if nums[i] == nums[j] and i < j.

• 1528. Shuffle String (solved here, tests)

  Description

  You are given a string s and an integer array indices of the same length. The string s will be shuffled such that the character at the i-th position moves to indices[i] in the shuffled string.

  Return the shuffled string.

• 1534. Count Good Triplets (solved here, tests)

  Description

  Given an array of integers arr, and three integers a, b and c. You need to find the number of good triplets.

  A triplet (arr[i], arr[j], arr[k]) is good if the following conditions are true:

  • 0 <= i < j < k < arr.length

  • |arr[i] - arr[j]| <= a

  • |arr[j] - arr[k]| <= b

  • |arr[i] - arr[k]| <= c

  Where |x| denotes the absolute value of x.

  Return the number of good triplets.

• 1572. Matrix Diagonal Sum (solved here, tests)

  Description

  Given a square matrix mat, return the sum of the matrix diagonals.

  Only include the sum of all the elements on the primary diagonal and all the elements on the secondary diagonal that are not part of the primary diagonal.

• 1588. Sum of All Odd Length Subarrays (solved here, tests)

  Description

  Given an array of positive integers arr, calculate the sum of all possible odd-length subarrays.

  A subarray is a contiguous subsequence of the array.

  Return the sum of all odd-length subarrays of arr.

• 1603. Design Parking System (solved here, tests)

  Description

  Design a parking system for a parking lot. The parking lot has three kinds of parking spaces: big, medium, and small, with a fixed number of slots for each size.

  Implement the ParkingSystem class:

  • ParkingSystem(int big, int medium, int small) Initializes object of the ParkingSystem class. The number of slots for each parking space are given as part of the constructor.
  • bool addCar(int carType) Checks whether there is a parking space of carType for the car that wants to get into the parking lot. carType can be of three kinds: big, medium, or small, which are represented by 1, 2, and 3 respectively. A car can only park in a parking space of its carType. If there is no space available, return false, else park the car in that size space and return true.

• 1614. Maximum Nesting Depth of the Parentheses (solved here, tests)

  Description

  A string is a valid parentheses string (denoted VPS) if it meets one of the following:

  • It is an empty string "", or a single character not equal to "(" or ")",

  • It can be written as AB (A concatenated with B), where A and B are VPS's, or

  • It can be written as (A), where A is a VPS.

  We can similarly define the nesting depth depth(S) of any VPS S as follows:

  • depth("") = 0

  • depth(C) = 0, where C is a string with a single character not equal to "(" or ")".

  • depth(A + B) = max(depth(A), depth(B)), where A and B are VPS's.

  • depth("(" + A + ")") = 1 + depth(A), where A is a VPS.

  For example, "", "()()", and "()(()())" are VPS's (with nesting depths 0, 1, and 2), and ")(" and "(()" are not VPS's.

  Given a VPS represented as string s, return the nesting depth of s.

• 1656. Design an Ordered Stream (solved here, tests)

  Description

  There is a stream of n (idKey, value) pairs arriving in an arbitrary order, where idKey is an integer between 1 and n and value is a string. No two pairs have the same id.

  Design a stream that returns the values in increasing order of their IDs by returning a chunk (list) of values after each insertion. The concatenation of all the chunks should result in a list of the sorted values.

  Implement the OrderedStream class:

  • OrderedStream(int n) Constructs the stream to take n values.
  • String[] insert(int idKey, String value) Inserts the pair (idKey, value) into the stream, then returns the largest possible chunk of currently inserted values that appear next in the order.

• 1662. Check If Two String Arrays are Equivalent (solved here, tests)

  Description

  Given two string arrays word1 and word2, return true if the two arrays represent the same string, and false otherwise.

  A string is represented by an array if the array elements concatenated in order forms the string.

• 1684. Count the Number of Consistent Strings (solved here, tests)

  Description

  You are given a string allowed consisting of distinct characters and an array of strings words. A string is consistent if all characters in the string appear in the string allowed.

  Return the number of consistent strings in the arraywords.

• 1688. Count of Matches in Tournament (solved here, tests)

  Description

  You are given an integer n, the number of teams in a tournament that has strange rules:

  If the current number of teams is even, each team gets paired with another team. A total of n / 2 matches are played, and n / 2 teams advance to the next round.

  If the current number of teams is odd, one team randomly advances in the tournament, and the rest gets paired. A total of (n - 1) / 2 matches are played, and (n - 1) / 2 + 1 teams advance to the next round.

  Return the number of matches played in the tournament until a winner is decided.

• 1704. Determine if String Halves Are Alike (solved here, tests)

  Description

  You are given a string s of even length. Split this string into two halves of equal lengths, and let a be the first half and b be the second half.

  Two strings are alike if they have the same number of vowels ('a', 'e', 'i', 'o', 'u', 'A', 'E', 'I', 'O', 'U'). Notice that s contains uppercase and lowercase letters.

  Return true if a and b are alike. Otherwise, return false.

• 1720. Decode XORed Array (solved here, tests)

  Description

  There is a hidden integer array arr that consists of n non-negative integers.

  It was encoded into another integer array encoded of length n - 1, such that encoded[i] = arr[i] XOR arr[i + 1]. For example, if arr = [1,0,2,1], then encoded = [1,2,3].

  You are given the encoded array. You are also given an integer first, that is the first element of arr, i.e. arr[0].

  Return the original array arr. It can be proved that the answer exists and is unique.

• 1773. Count Items Matching a Rule (solved here, tests)

  Description

  You are given an array items, where each items[i] = [type-i, color-i, name-i] describes the type, color, and name of the i-th item. You are also given a rule represented by two strings, ruleKey and ruleValue.

  The i-th item is said to match the rule if one of the following is true:

  ruleKey == "type" and ruleValue == type-i.

  ruleKey == "color" and ruleValue == color-i.

  ruleKey == "name" and ruleValue == name-i.

  Return the number of items that match the given rule.

• 1779. Find Nearest Point That Has the Same X or Y Coordinate (solved here, tests)

  Description

  You are given two integers, x and y, which represent your current location on a Cartesian grid: (x, y). You are also given an array points where each points[i] = [ai, bi] represents that a point exists at (ai, bi). A point is valid if it shares the same x-coordinate or the same y-coordinate as your location.

  Return the index (0-indexed) of the valid point with the smallest Manhattan distance from your current location. If there are multiple, return the valid point with the smallest index. If there are no valid points, return -1.

  The Manhattan distance between two points (x1, y1) and (x2, y2) is abs(x1 - x2) + abs(y1 - y2).

• 1790. Check if One String Swap Can Make Strings Equal (solved here, tests)

  Description

  You are given two strings s1 and s2 of equal length. A string swap is an operation where you choose two indices in a string (not necessarily different) and swap the characters at these indices.    Return true if it is possible to make both strings equal by performing at most one string swap on exactly one of the strings. Otherwise, return false. 

• 1791. Find Center of Star Graph (solved here, tests)

  Description

  There is an undirected star graph consisting of n nodes labeled from 1 to n. A star graph is a graph where there is one center node and exactly n - 1 edges that connect the center node with every other node.

  You are given a 2D integer array edges where each edges[i] = [ui, vi] indicates that there is an edge between the nodes ui and vi. Return the center of the given star graph.

• 1816. Truncate Sentence (solved here, tests)

  Description

  A sentence is a list of words that are separated by a single space with no leading or trailing spaces. Each of the words consists of only uppercase and lowercase English letters (no punctuation).

  • For example, "Hello World", "HELLO", and "hello world hello world" are all sentences.

  You are given a sentence s and an integer k. You want to truncate s such that it contains only the first k words. Return s after truncating it.

• 1832. Check if the Sentence Is Pangram (solved here, tests)

  Description

  A pangram is a sentence where every letter of the English alphabet appears at least once.

  Given a string sentence containing only lowercase English letters, return true if sentence is a pangram, or false otherwise.

• 1844. Replace All Digits with Characters (solved here, tests)

  Description

  You are given a 0-indexed string s that has lowercase English letters in its even indices and digits in its odd indices.

  There is a function shift(c, x), where c is a character and x is a digit, that returns the x-th character after c.

  • For example, shift('a', 5) = 'f' and shift('x', 0) = 'x'.

  For every odd index i, you want to replace the digit s[i] with shift(s[i-1], s[i]).

  Return s after replacing all digits. It is guaranteed that shift(s[i-1], s[i]) will never exceed 'z'.

• 1859. Sorting the Sentence (solved here, tests)

  Description

  A sentence is a list of words that are separated by a single space with no leading or trailing spaces. Each word consists of lowercase and uppercase English letters.

  A sentence can be shuffled by appending the 1-indexed word position to each word then rearranging the words in the sentence.

  For example, the sentence "This is a sentence" can be shuffled as "sentence4 a3 is2 This1" or "is2 sentence4 This1 a3". Given a shuffled sentence s containing no more than 9 words, reconstruct and return the original sentence.

• 1913. Maximum Product Difference Between Two Pairs (solved here, tests)

  Description

  The product difference between two pairs (a, b) and (c, d) is defined as (a * b) - (c * d).

  • For example, the product difference between (5, 6) and (2, 7) is (5 * 6) - (2 * 7) = 16.

  Given an integer array nums, choose four distinct indices w, x, y, and z such that the product difference between pairs (nums[w], nums[x]) and (nums[y], nums[z]) is maximized.

  Return the maximum such product difference.

• 1920. Build Array from Permutation (solved here, tests)

  Description

  Given a zero-based permutation nums (0-indexed), build an array ans of the same length where ans[i] = nums[nums[i]] for each 0 <= i < nums.length and return it.

  A zero-based permutation nums is an array of distinct integers from 0 to nums.length - 1 (inclusive).

• 1929. Concatenation of Array (solved here, tests)

  Description

  Given an integer array nums of length n, you want to create an array ans of length 2n where ans[i] == nums[i] and ans[i + n] == nums[i] for 0 <= i < n (0-indexed).

  Specifically, ans is the concatenation of two nums arrays.

  Return the array ans.

• 1967. Number of Strings That Appear as Substrings in Word (solved here, tests)

  Description

  Given an array of strings patterns and a string word, return the number of strings in patterns that exist as a substring in word.

  A substring is a contiguous sequence of characters within a string.

• 2006. Count Number of Pairs With Absolute Difference K (solved here, tests)

  Description

  Given an integer array nums and an integer k, return the number of pairs (i, j) where i < j such that |nums[i] - nums[j]| == k.

  The value of |x| is defined as:

  x if x >= 0. -x if x < 0.

• 2011. Final Value of Variable After Performing Operations (solved here, tests)

  Description

  There is a programming language with only four operations and one variable X:

  • ++X and X++ increments the value of the variable X by 1.

  • --X and X-- decrements the value of the variable X by 1.

  Initially, the value of X is 0.

  Given an array of strings operations containing a list of operations, return the final value of X after performing all the operations.

• 2037. Minimum Number of Moves to Seat Everyone (solved here, tests)

  Description

  There are n seats and n students in a room. You are given an array seats of length n, where seats[i] is the position of the i-th seat. You are also given the array students of length n, where students[j] is the position of the j-th student.

  You may perform the following move any number of times:

  Increase or decrease the position of the i-th student by 1 (i.e., moving the i-th student from position x to x + 1 or x - 1) Return the minimum number of moves required to move each student to a seat such that no two students are in the same seat.

  Note that there may be multiple seats or students in the same position at the beginning.

• 2103. Rings and Rods (solved here, tests)

  Description

  There are n rings and each ring is either red, green, or blue. The rings are distributed across ten rods labeled from 0 to 9.

  You are given a string rings of length 2n that describes the n rings that are placed onto the rods. Every two characters in rings forms a color-position pair that is used to describe each ring where:

  • The first character of the i-th pair denotes the i-th ring's color ('R', 'G', 'B').

  • The second character of the i-th pair denotes the rod that the i-th ring is placed on ('0' to '9').

  For example, "R3G2B1" describes n == 3 rings: a red ring placed onto the rod labeled 3, a green ring placed onto the rod labeled 2, and a blue ring placed onto the rod labeled 1.

  Return the number of rods that have all three colors of rings on them.

• 2114. Maximum Number of Words Found in Sentences (solved here, tests)

  Description

  A sentence is a list of words that are separated by a single space with no leading or trailing spaces.

  You are given an array of strings sentences, where each sentences[i] represents a single sentence.

  Return the maximum number of words that appear in a single sentence.

• 2160. Minimum Sum of Four Digit Number After Splitting Digits (solved here, tests)

  Description

  You are given a positive integer num consisting of exactly four digits. Split num into two new integers new1 and new2 by using the digits found in num. Leading zeros are allowed in new1 and new2, and all the digits found in num must be used.

  For example, given num = 2932, you have the following digits: two 2's, one 9 and one 3. Some of the possible pairs [new1, new2] are [22, 93], [23, 92], [223, 9] and [2, 329].

  Return the minimum possible sum of new1 and new2.

• 2176. Count Equal and Divisible Pairs in an Array (solved here, tests)

  Description

  Given a 0-indexed integer array nums of length n and an integer k, return the number of pairs (i, j) where 0 <= i < j < n, such that nums[i] == nums[j] and (i * j) is divisible by k.

• 2185. Counting Words With a Given Prefix (solved here, tests)

  Description

  You are given an array of strings words and a string pref.

  Return the number of strings in words that contain pref as a prefix.

  A prefix of a string s is any leading contiguous substring of s.

• 2194. Cells in a Range on an Excel Sheet (solved here, tests)

  Description

  A cell (r, c) of an excel sheet is represented as a string "<col><row>" where:

  -<col> denotes the column number c of the cell. It is represented by alphabetical letters.

  For example, the 1st column is denoted by 'A', the 2nd by 'B', the 3rd by 'C', and so on.

  -<row> is the row number r of the cell. The r-th row is represented by the integer r.

  You are given a string s in the format "<col1><row1>:<col2><row2>", where <col1> represents the column c1, <row1> represents the row r1, <col2> represents the column c2, and represents the row r2, such that r1 <= r2 and c1 <= c2.

  Return the list of cells (x, y) such that r1 <= x <= r2 and c1 <= y <= c2. The cells should be represented as strings in the format mentioned above and be sorted in non-decreasing order first by columns and then by rows.

• 2235. Add Two Integers (solved here, tests)

  Description

  Given two integers num1 and num2, return the sum of the two integers.

• 2236. Root Equals Sum of Children (solved here, tests)

  Description

  You are given the root of a binary tree that consists of exactly 3 nodes: the root, its left child, and its right child.

  Return true if the value of the root is equal to the sum of the values of its two children, or false otherwise.

medium lvl

• 2. Add Two Numbers (solved here, tests)

  Description

  You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order, and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list.

  You may assume the two numbers do not contain any leading zero, except the number 0 itself.

  Input: l1 = [2,4,3], l2 = [5,6,4]

  Output: [7,0,8]

  Explanation: 342 + 465 = 807

• 8. String to Integer (atoi) (solved here, tests)

  Description

  Implement the myAtoi(string s) function, which converts a string to a 32-bit signed integer (similar to C/C++'s atoi function).

  The algorithm for myAtoi(string s) is as follows:

  • Read in and ignore any leading whitespace.
  • Check if the next character (if not already at the end of the string) is '-' or '+'. Read this character in if it is either. This determines if the final result is negative or positive respectively. Assume the result is positive if neither is present.
  • Read in next the characters until the next non-digit character or the end of the input is reached. The rest of the string is ignored.
  • Convert these digits into an integer (i.e. "123" -> 123, "0032" -> 32). If no digits were read, then the integer is 0. Change the sign as necessary (from step 2).
  • If the integer is out of the 32-bit signed integer range [-2^31, 2^31 - 1], then clamp the integer so that it remains in the range. Specifically, integers less than -2^31 should be clamped to -2^31, and integers greater than 2^31 - 1 should be clamped to 2^31 - 1.
  • Return the integer as the final result.

  Note:

  Only the space character ' ' is considered a whitespace character. Do not ignore any characters other than the leading whitespace or the rest of the string after the digits.

• 29. Divide Two Integers (solved here, tests)

  Description

  Given two integers dividend and divisor, divide two integers without using multiplication, division, and mod operator.

  The integer division should truncate toward zero, which means losing its fractional part. For example, 8.345 would be truncated to 8, and -2.7335 would be truncated to -2.

  Return the quotient after dividing dividend by divisor.

  Note: Assume we are dealing with an environment that could only store integers within the 32-bit signed integer range: [−2^31, 2^31 − 1]. For this problem, if the quotient is strictly greater than 2^31 - 1, then return 2^31 - 1, and if the quotient is strictly less than -2^31, then return -2^31.

• 38. Count and Say (solved here, tests)

  Description

  The count-and-say sequence is a sequence of digit strings defined by the recursive formula:

  countAndSay(1) = "1" countAndSay(n) is the way you would "say" the digit string from countAndSay(n-1), which is then converted into a different digit string. To determine how you "say" a digit string, split it into the minimal number of groups so that each group is a contiguous section all of the same character. Then for each group, say the number of characters, then say the character. To convert the saying into a digit string, replace the counts with a number and concatenate every saying.

  For example, the saying and conversion for digit string "3322251":

  Given a positive integer n, return the n-th term of the count-and-say sequence.

• 535. Encode and Decode TinyURL (solved here, tests)

  Description

  TinyURL is a URL shortening service where you enter a URL such as https://leetcode.com/problems/design-tinyurl and it returns a short URL such as http://tinyurl.com/4e9iAk. Design a class to encode a URL and decode a tiny URL.

  There is no restriction on how your encode/decode algorithm should work. You just need to ensure that a URL can be encoded to a tiny URL and the tiny URL can be decoded to the original URL.

  Implement the Solution class:

  • Solution() Initializes the object of the system.

  • String encode(String longUrl) Returns a tiny URL for the given longUrl.

  • String decode(String shortUrl) Returns the original long URL for the given shortUrl. It is guaranteed that the given shortUrl was encoded by the same object.

• 1282. Group the People Given the Group Size They Belong To (solved here, tests)

  Description

  There are n people that are split into some unknown number of groups. Each person is labeled with a unique ID from 0 to n - 1.

  You are given an integer array groupSizes, where groupSizes[i] is the size of the group that person i is in. For example, if groupSizes[1] = 3, then person 1 must be in a group of size 3.

  Return a list of groups such that each person i is in a group of size groupSizes[i].

  Each person should appear in exactly one group, and every person must be in a group. If there are multiple answers, return any of them. It is guaranteed that there will be at least one valid solution for the given input.

• 1329. Sort the Matrix Diagonally (solved here, tests)

  Description

  A matrix diagonal is a diagonal line of cells starting from some cell in either the topmost row or leftmost column and going in the bottom-right direction until reaching the matrix's end. For example, the matrix diagonal starting from mat[2][0], where mat is a 6 x 3 matrix, includes cells mat[2][0], mat[3][1], and mat[4][2].

  Given an m x n matrix mat of integers, sort each matrix diagonal in ascending order and return the resulting matrix.

• 1409. Queries on a Permutation With Key (solved here, tests)

  Description

  Given the array queries of positive integers between 1 and m, you have to process all queries[i] (from i=0 to i=queries.length-1) according to the following rules:

  • In the beginning, you have the permutation P=[1,2,3,...,m].

  • For the current i, find the position of queries[i] in the permutation P (indexing from 0) and then move this at the beginning of the permutation P. Notice that the position of queries[i] in P is the result for queries[i].

  Return an array containing the result for the given queries.

• 1476. Subrectangle Queries (solved here, tests)

  Description

  Implement the class SubrectangleQueries which receives a rows x cols rectangle as a matrix of integers in the constructor and supports two methods:

  1. updateSubrectangle(int row1, int col1, int row2, int col2, int newValue)

  Updates all values with newValue in the subrectangle whose upper left coordinate is (row1,col1) and bottom right coordinate is (row2,col2).

  2. getValue(int row, int col)

  Returns the current value of the coordinate (row,col) from the rectangle.

• 1769. Minimum Number of Operations to Move All Balls to Each Box (solved here, tests)

  Description

  You have n boxes. You are given a binary string boxes of length n, where boxes[i] is '0' if the i-th box is empty, and '1' if it contains one ball.

  In one operation, you can move one ball from a box to an adjacent box. Box i is adjacent to box j if abs(i - j) == 1. Note that after doing so, there may be more than one ball in some boxes.

  Return an array answer of size n, where answer[i] is the minimum number of operations needed to move all the balls to the i-th box.

  Each answer[i] is calculated considering the initial state of the boxes.

• 1828. Queries on Number of Points Inside a Circle (solved here, tests)

  Description

  You are given an array points where points[i] = [xi, yi] is the coordinates of the i-th point on a 2D plane. Multiple points can have the same coordinates.

  You are also given an array queries where queries[j] = [xj, yj, rj] describes a circle centered at (xj, yj) with a radius of rj.

  For each query queries[j], compute the number of points inside the j-th circle. Points on the border of the circle are considered inside.

  Return an array answer, where answer[j] is the answer to the j-th query.

• 2079. Watering Plants (solved here, tests)

  Description

  You want to water n plants in your garden with a watering can. The plants are arranged in a row and are labeled from 0 to n - 1 from left to right where the i-th plant is located at x = i. There is a river at x = -1 that you can refill your watering can at.

  Each plant needs a specific amount of water. You will water the plants in the following way:

  • Water the plants in order from left to right.

  • After watering the current plant, if you do not have enough water to completely water the next plant, return to the river to fully refill the watering can.

  • You cannot refill the watering can early.

  You are initially at the river (i.e., x = -1). It takes one step to move one unit on the x-axis.

  Given a 0-indexed integer array plants of n integers, where plants[i] is the amount of water the i-th plant needs, and an integer capacity representing the watering can capacity, return the number of steps needed to water all the plants.

• 2120. Execution of All Suffix Instructions Staying in a Grid (solved here, tests)

  Description

  There is an n x n grid, with the top-left cell at (0, 0) and the bottom-right cell at (n - 1, n - 1). You are given the integer n and an integer array startPos where startPos = [startrow, startcol] indicates that a robot is initially at cell (startrow, startcol).

  You are also given a 0-indexed string s of length m where s[i] is the i-th instruction for the robot: 'L' (move left), 'R' (move right), 'U' (move up), and 'D' (move down).

  The robot can begin executing from any i-th instruction in s. It executes the instructions one by one towards the end of s but it stops if either of these conditions is met:

  • The next instruction will move the robot off the grid.
  • There are no more instructions left to execute.

  Return an array answer of length m where answer[i] is the number of instructions the robot can execute if the robot begins executing from the i-th instruction in s.

• 2125. Number of Laser Beams in a Bank (solved here, tests)

  Description

  Anti-theft security devices are activated inside a bank. You are given a 0-indexed binary string array bank representing the floor plan of the bank, which is an m x n 2D matrix. bank[i] represents the i-th row, consisting of '0's and '1's. '0' means the cell is empty, while '1' means the cell has a security device.

  There is one laser beam between any two security devices if both conditions are met:

  • The two devices are located on two different rows: r1 and r2, where r1 < r2.

  • For each row i where r1 < i < r2, there are no security devices in the i-th row.

  Laser beams are independent, i.e., one beam does not interfere nor join with another.

  Return the total number of laser beams in the bank.

• 2149. Rearrange Array Elements by Sign (solved here, tests)

  Description

  You are given a 0-indexed integer array nums of even length consisting of an equal number of positive and negative integers.

  You should rearrange the elements of nums such that the modified array follows the given conditions:

  Every consecutive pair of integers have opposite signs. For all integers with the same sign, the order in which they were present in nums is preserved. The rearranged array begins with a positive integer. Return the modified array after rearranging the elements to satisfy the aforementioned conditions.

• 2161. Partition Array According to Given Pivot (solved here, tests)

  Description

  You are given a 0-indexed integer array nums and an integer pivot. Rearrange nums such that the following conditions are satisfied:

  • Every element less than pivot appears before every element greater than pivot.
  • Every element equal to pivot appears in between the elements less than and greater than pivot. The relative order of the elements less than pivot and the elements greater than pivot is maintained. More formally, consider every pi, pj where pi is the new position of the i-th element and pj is the new position of the j-th element. For elements less than pivot, if i < j and nums[i] < pivot and nums[j] < pivot, then pi < pj. Similarly for elements greater than pivot, if i < j and nums[i] > pivot and nums[j] > pivot, then pi < pj. Return nums after the rearrangement.

hard lvl

• 4. Median of Two Sorted Arrays (solved here, tests)

  Description

  Given two sorted arrays nums1 and nums2 of size m and n respectively, return the median of the two sorted arrays.

  The overall run time complexity should be O(log (m+n)).