LeetCode-Sum Root to Leaf Numbers

/**
* 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) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
return sumNum(root, 0);
}

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