Convert Sorted List to Binary Search Tree

public static TreeNode sortedListToBST( ListNode head ) {
    if( head == null ) return null;
    if( head.next == null ) return new TreeNode( head.val );

    ListNode dummyHead = new ListNode( 0 );
    dummyHead.next = head;
    ListNode slow = head, fast = head.next;
    ListNode tail = dummyHead;
    while( fast != null && fast.next != null ) {
        tail = slow;
        slow = slow.next;
        fast = fast.next.next;
    }
    tail.next = null;
    TreeNode root = new TreeNode( slow.val );
    root.left = sortedListToBST( dummyHead.next );
    root.right = sortedListToBST( slow.next );
    return root;
}