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   7

 

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]
解题思路：当树的高度等于ans.size()的时候，说明是第一次到达depth深度，此时需要新建一个一维的vector

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