129. 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.

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