方法（DFS）
    遇到问题vector的反转reverse(levelOrder.begin(), levelOrder.end())

    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>> levelOrderBottom(TreeNode* root) {
    15.         dsf(root,0);
    16.         reverse(s.begin(), s.end());
    17.         return s;
    18.     }
    19.     void dsf(TreeNode* root,int depth)
    20.     {
    21.         if(root==NULL){
    22.             return;
    23.         }
    24.         if(s.size()==depth){
    25.             s.emplace_back();
    26.         }
    27.         s[depth].emplace_back(root->val);
    28.         dsf(root->left,depth+1);
    29.         dsf(root->right,depth+1);
    30.     }
    31.     vector<vector<int>> s;
    32. };