102. Binary Tree Level Order Traversal

Given a binary tree, return the level order traversal of its nodes’ values. (ie, from left to right, level by level).

For example: Given binary tree [3,9,20,null,null,15,7],

3 / \ 9 20 / \ 15 7

return its level order traversal as:

[ [3], [9,20], [15,7] ]

Solution C++

/** * Definition for a binary tree node. * struct TreeNode { * int val; * TreeNode *left; * TreeNode *right; * TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */ class Solution { public: vector<vector<int>> levelOrder(TreeNode* root) { vector<vector<int> > res; if(root == NULL) return res; queue<TreeNode*> q; q.push(root); while(!q.empty()) { vector<int> cur; int n = q.size(); for(int i = 0; i < n; i++) { TreeNode* node = q.front(); cur.push_back(node->val); if(node->left) q.push(node->left); if(node->right) q.push(node->right); q.pop(); } res.push_back(cur); } return res; } };

Explanation