82. Remove Duplicates from Sorted List II

去掉有序链表中的全部重复元素，如[1,1,1,2,3] -> [2,3]。设置dummy node，双指针跳过重复node

public ListNode deleteDuplicates(ListNode head) {
    ListNode dummy = new ListNode(0);
    dummy.next = head;
    ListNode pre = dummy;

    while (head != null) {
        while (head.next != null && head.val == head.next.val) {
            head = head.next;
        }
        if (pre.next != head) {   // there are some duplicated values in between
            pre.next = head.next;
        } else {                  // no duplicate values
            pre = pre.next;
        }
        head = head.next;
    }
    return dummy.next;
}