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