[Leetcode]Binary Tree Level Order Traversal

/** * 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) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        vector<vector<int> > ret;        vector<int> ans;        if(root == NULL) return ret;         queue<pair<TreeNode*, int> > q;        q.push(make_pair(root, 0));        int curLevel = 0;        while(!q.empty())        {            int level = q.front().second;            TreeNode *node = q.front().first;            q.pop();                        if(level != curLevel)            {                curLevel++;                ret.push_back(ans);                ans.clear();            }            ans.push_back(node->val);                        if(node->left != NULL)                q.push(make_pair(node->left, level+1));            if(node->right != NULL)                q.push(make_pair(node->right, level+1));        }        ret.push_back(ans);        return ret;    }};