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