方法：DFS

    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.     int totalsum=0;
    15.     int ret=0;
    16.     int sumNumbers(TreeNode* root) {
    17.         if(root==NULL){
    18.             return 0;
    19.         }
    20.         ret=ret*10+root->val;
    21.         if(!root->left&&!root->right)
    22.         {
    23.             totalsum+=ret;
    24.         }
    25.         sumNumbers(root->left);
    26.         sumNumbers(root->right);
    27.         ret=ret/10;
    28.         return totalsum;
    29.     }
    30. };