Please help! Thank you!

```
static int sum;
public int sumNumbers(TreeNode root) {
if(root == null) return 0;
helper(root,0);
return this.sum;
}
public void helper(TreeNode node, int n){
n = n * 10 + node.val;
if(node.left == null && node.right == null){
this.sum = this.sum + n;
return;
}
if(node.left != null) helper(node.left,n);
if(node.right != null) helper(node.right,n);
}
Input:[0,1]
Output:10
Expected:1
```