与429题相同
    注意：vector> 需要使用if(result.size()==depth){ result.emplace_back();来预先准备一个空数组

    1. /**
    2.  * Definition for a binary tree node.
    3.  * struct TreeNode {
    4.  *     int val;
    5.  *     TreeNode *left;
    6.  *     TreeNode *right;
    7.  *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
    8.  *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
    9.  *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
    10.  * };
    11.  */
    12. class Solution {
    13. public:
    14.     vector<vector<int>> levelOrder(TreeNode* root) {
    15.         dfs(root,0);
    16.         return result;
    17.     }
    18.     void dfs(TreeNode* root,int depth){
    19.         if(root==NULL){
    20.             return;
    21.         }
    22.         if(result.size()==depth){
    23.             result.emplace_back();
    24.         }
    25.         result[depth].emplace_back(root->val);
    26.         dfs(root->left,depth+1);
    27.         dfs(root->right,depth+1);
    28.     }
    29. private:
    30.     vector<vector<int>>result;
    31. };