算法描述：

Given a binary tree containing digits from 0-9 only, each root-to-leaf path could represent a number.

An example is the root-to-leaf path 1->2->3 which represents the number 123.

Find the total sum of all root-to-leaf numbers.

Note: A leaf is a node with no children.

Example:

Input: [1,2,3]
    1
   /   2   3
Output: 25
Explanation:
The root-to-leaf path 1->2
 
 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.
Therefore, sum = 12 + 13 = 25.

Example 2:

Input: [4,9,0,5,1]
    4
   /   9   0
 / 5   1
Output: 1026
Explanation:
The root-to-leaf path 4->9->5 represents the number 495.
The root-to-leaf path 4->9->1 represents the number 491.
The root-to-leaf path 4->0
 
 represents the number 40.
Therefore, sum = 495 + 491 + 40 = 1026.

解題思路：用輔助變量存儲相加結果。當前節點為葉子節點時返回結果，否則繼續叠代相加。

    int sumNumbers(TreeNode* root) {
        return sumDsf(root, 0);
    }
    
    int sumDsf(TreeNode* root, int sumValue){
        if(root == nullptr) return 0;
        sumValue = sumValue*10 + root->val;
        
 
if(root->left==nullptr && root->right==nullptr)
            return sumValue;
        return sumDsf(root->left,sumValue) + sumDsf(root->right,sumValue);
        
    }

LeetCode-129-Sum Root to Leaf Numbers