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