leetcode82~Remove Duplicates from Sorted List II

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;
    }
}