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 ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param head ListNode类
     * @return ListNode类
     */
    public ListNode deleteDuplicates (ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        //当前元素
        ListNode current = head;
        //下一个元素
        ListNode nextNode;

        //返回的结果集
        ListNode resultNode = null;

        //上一个元素
        ListNode preNode = null;
        while (current != null) {
            //往前走找，直到第一个不相等的值为止
            nextNode = current.next;
            int count = 0;
            while (nextNode != null && nextNode.val == current.val) {
                count++;
                nextNode = nextNode.next;
            }
            if (count == 0) {
                if (preNode != null) {
                    preNode.next = current;
                }
                if (resultNode == null) {
                    resultNode = current;
                }
                preNode = current;
                current = current.next;
            } else {
                if (preNode != null) {
                    preNode.next = null;
                }
                current = nextNode;
            }
        }
        return resultNode;
    }
}