class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        if (head == null) {
            return null;
        }
        ListNode p = head;
        ListNode q = head;
        int data = q.val;
        p = p.next;
        while (p != null) {
            if ( p.val == data) {
                q.next = p.next;
                p = p.next;
            } else {
                q = q.next;
                p = p.next;
                data = q.val;
            }
        }
        return head;
    }
}