129. 求根节点到叶节点数字之和

 深搜，每下一层×10，直到到了叶节点才返回值，

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int DFS(TreeNode* root,int pre)
    {
        if(!root)
        {
            return 0;
        }
        int sum=10*pre+root->val;
        if(!root->left && !root->right)
        {
            return sum;
        }
        return DFS(root->left,sum)+DFS(root->right,sum);
    }
    int sumNumbers(TreeNode* root) {
       return DFS(root,0);
    }
    
};