82. Remove Duplicates from Sorted List II

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        if(head == null) return null;
        ListNode dummy = new ListNode(0);
        ListNode prev = dummy;
        prev.next = head;
        ListNode cur = head;
        while(cur!=null && cur.next != null){//缺一不可
            boolean dup = false;
            while(cur.next!=null &&cur.val == cur.next.val){//缺一不可
                cur = cur.next;
                dup = true;
            }
            if(dup){
                prev.next = cur.next;
            }
            else{
                prev = cur;
            }
            cur = cur.next;
        }
        return dummy.next;
    }
}