https://www.cnblogs.com/grandyang/p/4273700.html

sum代表当前节点的和。

这个题要从上往下的思路去做。

class Solution {public:    int sumNumbers(TreeNode* root) {        return sumNumbers(root,0);    }    int sumNumbers(TreeNode* root,int sum){        if(!root)            return 0;        sum = sum*10 + root->val;        if(!root->left && !root->right)            return sum;        return sumNumbers(root->left,sum) + sumNumbers(root->right,sum);    }};