82. 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.

Example 1:

Input: 1->2->3->3->4->4->5
Output: 1->2->5

Example 2:

Input: 1->1->1->2->3
Output: 2->3

因为需要移走所有重复的元素，可能head会被移除，因而需要一个dummy node

time: O(n), space: O(1)

/**
 * 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 || head.next == null) {
            return head;
        }
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode cur = dummy;
        while(cur.next != null && cur.next.next != null) {
            if(cur.next.val == cur.next.next.val) {
                ListNode tmp = cur.next;
                while(tmp != null && tmp.val == cur.next.val) {
                    tmp = tmp.next;
                }
                cur.next = tmp;
            } else {
                cur = cur.next;
            }
        }
        return dummy.next;
    }
}