LeetCode 102: Binary Tree Level Order Traversal

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,#,#,15,7}

return its level order traversal as:

解题思路

考察二叉树的层次遍历，利用队列（queue）很容易就可以实现该功能。代码如下：

/** * 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>> result;        if (root == NULL) return result;        queue<TreeNode *> nodeQueue;        nodeQueue.push(root);        while (!nodeQueue.empty()) {            vector<int> level_values;            int level_size = nodeQueue.size(); // Note: 判断每层的元素个数            for (int i = 0; i < level_size; ++i) {                TreeNode *front = nodeQueue.front();                nodeQueue.pop();                level_values.push_back(front->val);                if (front->left != NULL) nodeQueue.push(front->left);                if (front->right != NULL) nodeQueue.push(front->right);            }            result.push_back(level_values);        }        return result;    }};