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.

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