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

		sum_left = dfs(root->left, sum * 10 + root->val);

		sum_right = dfs(root->right, sum * 10 + root->val);
		return sum_left+sum_right;
	}
};