Binary Tree Level Order Traversal

一、问题描述

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]]

二、思路

按照层次递归遍历

三、代码

/** * 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> > ret;    void buildVector(TreeNode *root,int depth){        if(root == NULL) return;        if(ret.size() == depth)            ret.push_back(vector<int>());        ret[depth].push_back(root -> val);        buildVector(root -> left,depth + 1);        buildVector(root -> right,depth + 1);    }    vector<vector<int>> levelOrder(TreeNode *root) {        buildVector(root,0);        return ret;    }};