Leetcode Binary Tree Level Order Traversal

层次遍历，把每一层结点的数存在一个vector里面。bfs，用两个队列，一个队列存结点，一个队列存层数。

看到网上的一些题解，还可以用dfs做，NB。

/** * Definition for binary tree * 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) {        // Note: The Solution object is instantiated only once and is reused by each test case.        vector<vector<int> > ans;        vector<int> v;        if(root == NULL)    return ans;        queue<TreeNode*> node;        queue<int> index;        node.push(root);        index.push(1);        TreeNode *n;        int i;        int tag = 1;        while(!node.empty()){                        n = node.front();            i = index.front();            node.pop();            index.pop();            if(n->left != NULL){                node.push(n->left);                index.push(i+1);            }               if(n->right != NULL){                node.push(n->right);                index.push(i+1);            }            if(tag != i){                tag = i;                ans.push_back(v);                v.clear();            }            v.push_back(n->val);        }        ans.push_back(v);        return ans;    }};