Remove Duplicates from Sorted List II leetcode

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.

因为表头可能被删除， 创建一个dummy node， 令dummynode.next = head, return dummy.next

时间O(n) 空间O(1)

 
public class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode cur = head;
        ListNode pre = dummy;
        while (cur != null && cur.next != null) {
            if (cur.val == cur.next.val) {
                int val = cur.val;
                while (cur != null && cur.val == val) {
                    cur = cur.next;
                }
                pre.next = cur;
            } else {
                pre = pre.next;
                cur = cur.next;
            }
        }
        return dummy.next;
    }
}

public class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        if (head == null || head.next == null){
            return head;
        }
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        head = dummy;
        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 {//注意一定要在else语句里head往下传递 否则会溢出
                head = head.next;
            }
        }
        return dummy.next;
    }
}