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   7return its level order traversal as:[  [3],  [9,20],  [15,7]]使用一个队列来实现层次遍历*//** * 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) return res;        queue<TreeNode*> q;        q.push(root);        while(!q.empty())        {            int len=q.size();            vector<int> tmp;            for(int i=0;i<len;i++)            {                TreeNode* node=q.front();                tmp.push_back(node->val);                q.pop();                if(node->left) q.push(node->left);                if(node->right) q.push(node->right);            }            res.push_back(tmp);        }        return res;    }   };