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.

For example,

    1
   / \
  2   3

The root-to-leaf path 1->2

Return the sum = 12 + 13 = 25.

這道求根到葉節點數字之和的題跟之前的求Path Sum 二叉樹的路徑和很類似，都是利用DFS遞迴來解，這道題由於不是單純的把各個節點的數字相加，而是每到一個新的數字，要把原來的數字擴大10倍之後再相加。程式碼如下：

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