Convert BST to Sorted Doubly Linked List (Java)

Node prev = null;
Node head = null;
 
public Node treeToDoublyList(Node root) {
    if (root == null) {
        return null;
    }
 
    helper(root);
 
    head.left = prev;
    prev.right = head;
 
    return head;
}
 
private void helper(Node p) {
    if (p == null) {
        return;
    }
 
    helper(p.left);
 
    //handle current
    if (prev == null) {
        head = p;
    } else {
        prev.right = p;
        p.left = prev;
    }
 
    prev = p;
 
    helper(p.right);
}