Sum Root to Leaf Numbers

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