leetcode82~Remove Duplicates from Sorted List II

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Given a sorted linked list, delete all nodes that have duplicate numbers, leaving only distinct numbers from the original list.
For example,
Given 1->2->3->3->4->4->5, return 1->2->5.
Given 1->1->1->2->3, return 2->3.

本质是三个指针，一个是记录重复数字之前的指针，一个是当前指针，一个是重复数字之后的指针

public class RemoveDuplicatesfromSortedListII {    public ListNode deleteDuplicates2(ListNode head) {        if(head==null || head.next==null) return head;        ListNode dummyNode = new ListNode(-1);        dummyNode.next = head;        ListNode pre = dummyNode;        ListNode cur = head;        while(cur!=null) {            while(cur.next!=null && cur.val==cur.next.val) {                cur = cur.next;            }            if(pre.next==cur) {                pre = pre.next;            } else {                pre.next = cur.next;            }            cur = cur.next;        }        return dummyNode.next;    }    public ListNode deleteDuplicates(ListNode head) {        if(head==null || head.next==null) {            return head;        }        ListNode dummyNode = new ListNode(-1);        dummyNode.next = head;        head = dummyNode;        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 dummyNode.next;    }}

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