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ThreeSum.java
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ThreeSum.java
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package threeSum;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
/**
* @author shengfei
* # Leetcode #15, Medium
*/
public class ThreeSum {
public static void main(String[] args) {
List<List<Integer>> resultList = ThreeSum.threeSum(new int[]{-1, 0, 1, 2, -1, -4});
System.out.println(resultList);
resultList = ThreeSum.threeSum(new int[]{-4, -1, -1, 0, 1, 2});
System.out.println(resultList);
resultList = ThreeSum.threeSum(new int[]{-4, -1, -1, -1, 0, 1, 2});
System.out.println(resultList);
resultList = ThreeSum.threeSum(new int[]{-4, -1, -1, 0, 1, 1, 2});
System.out.println(resultList);
resultList = ThreeSum.threeSum(new int[]{-4, -1, -1});
System.out.println(resultList);
}
public static List<List<Integer>> threeSum(int[] nums) {
if (nums == null) {
return null;
}
if (nums.length < 3) {
return new ArrayList<>();
}
Arrays.sort(nums);
List<List<Integer>> resultList = new ArrayList<>();
for (int i = 0; i < nums.length - 2; i++) {
if (nums[i] > 0) {
break;
}
if ((i == 0) || (i > 0 && nums[i] != nums[i -1])) {
int j = i + 1;
int k = nums.length - 1;
while (j < k) {
if (nums[i] + nums[j] + nums[k] == 0) {
List<Integer> itemList = new ArrayList<>();
itemList.add(nums[i]);
itemList.add(nums[j]);
itemList.add(nums[k]);
resultList.add(itemList);
while (j < k && nums[j+1] == nums[j]) {
j++;
}
while (j < k && nums[k-1] == nums[k]) {
k--;
}
j++;
k--;
} else if (nums[i] + nums[j] + nums[k] > 0) {
k--;
} else {
j++;
}
}
}
}
return resultList;
}
/**
* 时间复杂度O(n^2), 空间复杂度O(1)
*/
public static List<List<Integer>> threeSum3(int[] nums) {
if (nums == null) {
return null;
}
if (nums.length < 3) {
return new ArrayList<>();
}
Arrays.sort(nums);
Set<List<Integer>> resultSet = new HashSet<>();
for (int i = 0; i < nums.length - 2; i++) {
if (nums[i] > 0) {
break;
}
int j = i + 1;
int k = nums.length - 1;
while (j < k) {
if (nums[i] + nums[j] + nums[k] == 0) {
List<Integer> itemList = new ArrayList<>();
itemList.add(nums[i]);
itemList.add(nums[j]);
itemList.add(nums[k]);
resultSet.add(itemList);
j++;
k--;
} else if (nums[i] + nums[j] + nums[k] > 0) {
k--;
} else {
j++;
}
}
}
List<List<Integer>> resultList = new ArrayList<>(resultSet);
return resultList;
}
/**
* 时间复杂度O(n^2), 空间复杂度O(n)
*/
public static List<List<Integer>> threeSum2(int[] nums) {
if (nums == null) {
return null;
}
if (nums.length < 3) {
return new ArrayList<>();
}
Arrays.sort(nums);
Set<List<Integer>> resultSet = new HashSet<>();
Set<Integer> set;
for (int i = 0; i < nums.length - 2; i++) {
if (nums[i] > 0) {
break;
}
set = new HashSet<>();
int left = 0 - nums[i];
for (int j = i + 1; j < nums.length; j++) {
if (set.contains(left - nums[j])) {
List<Integer> itemList = new ArrayList<>();
itemList.add(nums[i]);
itemList.add(left - nums[j]);
itemList.add(nums[j]);
resultSet.add(itemList);
}
set.add(nums[j]);
}
}
List<List<Integer>> resultList = new ArrayList<>(resultSet);
return resultList;
}
}