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 / \

class Solution {  
public:  
    vector<vector<int>> levelOrder(TreeNode* root)   
    {  
          
    vector<vector<int>> res;  
    if (root == NULL)  
    {  
        return res;  
    }
    queue<TreeNode*,int>> q;  
    q.push(make_pair(root,0));
    while(!q.empty()){
    
         TreeNode* node=q.front().first;
         int level=q.front().second;
         q.pop();
        if(level==res.size())
             res.push_back(vector<int>());
        res[level].push_back(node->val);
       if(node->left)
        q.push(make_pair(node->left,level+1));
     if(node->right)
        q.push(make_pair(node->right,level+1));
      }
    return res;
  }
};