题解 | #删除有序链表中重复的元素-II#

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 * }
 */

public class Solution {
    /**
     *
     * @param head ListNode类
     * @return ListNode类
     */
    public ListNode deleteDuplicates (ListNode head) {
        // write code here
        if (head == null || head.next == null) {
            return head;
        }
        ListNode dummy = new ListNode(-1);
        dummy.next = head;
        ListNode slow = dummy;
        ListNode fast = head;
        while (fast != null && fast.next != null) {
            if (fast.val == fast.next.val) {
                int tmp = fast.val;
                while (fast != null  && tmp == fast.val) {
                    fast = fast.next;
                }
            } else {
                slow.next = fast;
                slow = fast; 
                if (fast != null) {
                    fast = fast.next;
                }   
            }
            
        }
        slow.next = fast;
        

        return dummy.next;
    }


    public ListNode deleteDuplicates2 (ListNode head) {
        // write code here
        if (head == null || head.next == null) {
            return head;
        }
        Map<Integer, Integer> map = new HashMap<>();
        ListNode dummy = new ListNode(-1);
        ListNode tail = dummy;
        ListNode cur = head;
        while (cur != null) {
            map.put(cur.val, map.getOrDefault(cur.val, 0) + 1);
            cur = cur.next;
        }
        cur = head;
        while (cur != null) {
            if (map.get(cur.val) == 1) {
                tail.next = cur;
                tail = cur;
            }
            cur = cur.next;
        }
        tail.next = null;
        return dummy.next;
    }

}