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Graph Valid Tree.cpp
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Graph Valid Tree.cpp
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/*
Given n nodes labeled from 0 to n - 1 and a list of undirected edges
(each edge is a pair of nodes), write a function to check whether
these edges make up a valid tree.
Link: http://www.lintcode.com/en/problem/graph-valid-tree/#
Example: Given n = 5 and edges = [[0, 1], [0, 2], [0, 3], [1, 4]], return true.
Solution: the key step is to find the cycle in this tree, and if there no cycle,
result is true.
Source from: https://github.com/kamyu104/LintCode/blob/master/C++/graph-valid-tree.cpp
*/
class Solution {
public:
struct node {
int parent = -1;
vector<int> neighbors;
};
bool validTree(int n, vector<vector<int>>& edges) {
if (edges.size() != n - 1) {
return false;
} else if (n == 1) {
return true;
}
/*
unordered_map<Key,T>::iterator it;
(*it).first; // the key value (of type Key)
(*it).second; // the mapped value (of type T)
(*it); // the "element value" (of type pair<const Key,T>)
*/
unordered_map<int, node> nodes;
for (const auto& edge : edges) {
// emplace: insert a node at end of vector
nodes[edge.front()].neighbors.emplace_back(edge.back());
nodes[edge.back()].neighbors.emplace_back(edge.front());
// front() Returns a reference to the first element in the containers.
// back() Returns a reference to the last element in the containers.
}
if (nodes.size() != n) {
return false;
}
unordered_set<int> visited;
queue<int> q;
q.emplace(0);
while (!q.empty()) {
const int i = q.front();
q.pop();
visited.emplace(i);
for (const auto& node : nodes[i].neighbors) {
if (node != nodes[i].parent) {
if (visited.find(node) != visited.end()) {
return false;
} else {
// emplace: insert a node
visited.emplace(node);
nodes[node].parent = i;
q.emplace(node);
}
}
}
}
return visited.size() == n;
}
};