LeetCode 101：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},

    3   / \  9  20    /  \   15   7

return its level order traversal as:

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

题目要求对二叉树进行层次遍历，代码如下：

class Solution{public:    vector<vector<int> > levelOrder(TreeNode *root)    {        vector<vector<int>>result;        if (!root)            return result;        queue<TreeNode*> vec;        vec.push(root);        while (vec.size() > 0) {            queue<TreeNode*> tmp_vec;            vector<int> tmp_result;            while (vec.size() > 0) {                TreeNode* node = vec.front();                vec.pop();                if (node->left)                    tmp_vec.push(node->left);                if (node->right)                    tmp_vec.push(node->right);                tmp_result.push_back(node->val);            }            vec = tmp_vec;            result.push_back(tmp_result);        }        return result;    }};