remove duplicates from sorted list i, ii

public ListNode (ListNode head) {
    if (head == null || head.next == null)
        return head;
    ListNode ptr = head;
    while (ptr != null && ptr.next != null) {
        if (ptr.val == ptr.next.val) {
            ptr.next = ptr.next.next;
        } else {
            ptr = ptr.next;
        }
    }
    return head;
}

public ListNode (ListNode head) {
    if (head == null || head.next == null)
        return head;
    ListNode dummy = new ListNode(-1);
    dummy.next = head;
    head = dummy;
    while (head.next != null && head.next.next != null) {
        if (head.next.val == head.next.next.val) {
            int val = head.next.val;
            while (head.next != null && head.next.val == val) {
                head.next = head.next.next;
            }
        } else {
            head = head.next;
        }
    }
    return dummy.next;
}