Sum Root to Leaf Numbers

/**
* Definition for binary tree
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public int sumNumbers(TreeNode root) {
// Start typing your Java solution below
// DO NOT write main() function
return sum(root,0);
}
private int sum(TreeNode node,int num){
if(node==null)return 0;
int cur = num*10 + node.val;
if(node.left!=null||node.right!=null){
return sum(node.left,cur)+sum(node.right,cur);
}
else return cur;
}
}