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]]

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主要是把每层的数据保存在vector中，层次遍历的方法，层次遍历是一个节点一个节点进行循环的，一个节点可能有两个子节点，主要是一下去把子节点弄到队列里。

/** * 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>p;            int _size = q.size();            for(int i = 0; i < _size; i++){            TreeNode *temp = q.front();            q.pop();            p.push_back(temp->val);            if(temp->left)                q.push(temp->left);            if(temp->right)                q.push(temp->right);            }            res.push_back(p);        }         return res;    }};