Sum Root to Leaf Numbers

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 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.

Return the sum = 12 + 13 = 25.

C++:
01 void sumNumbersHelper(TreeNode *root, int& sum, int current)
02  {
03      if(root==NULL)
04          return;
05      int n=10*current+root->val;
06      if((root->left==NULL)&&(root->right==NULL))
07      {
08          sum+=n;
09          return;
10      }
11      sumNumbersHelper(root->left,sum,n);
12      sumNumbersHelper(root->right,sum,n);
13  }
14 
15 class Solution {
16 public:
17     int sumNumbers(TreeNode *root) {
18         // Start typing your C/C++ solution below
19         // DO NOT write int main() function
20         int result=0;
21         sumNumbersHelper(root,result,0);
22         return result;
23     }
24 };
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