栈和队列----删除无序单链表中值重复出现的节点

package com.test;

import com.test.ListNode;

import java.util.HashSet;

/**
 * Created by Demrystv.
 */
public class RemoveRepeat {

    /**
     * 方法一：利用哈希表，其时间复杂度是 O(N)，空间复杂度是 O(N)
     */
    public void removeRepeat1(ListNode head){
        if (head == null){
            return;
        }
        HashSet<Integer> hashSet = new HashSet<Integer>();
        ListNode pre = head;
        ListNode cur = head.next;
        hashSet.add(head.val);
        while (cur != null){
            if (hashSet.contains(cur)){
                pre.next = cur.next;
            }else {
                hashSet.add(cur.val);
                pre = cur;
            }
            cur = cur.next;
        }
    }


    // 方法二：类似选择排序的过程，时间复杂度是O(N^2),空间复杂度是O(1)
    public void removeRep2(ListNode head) {
        ListNode cur = head;
        ListNode pre = null;
        ListNode next = null;
        while (cur != null) {
            pre = cur;
            next = cur.next;
            while (next != null) {
                if (cur.val == next.val) {
                    pre.next = next.next;
                } else {
                    pre = next;
                }
                next = next.next;
            }
            cur = cur.next;
        }
    }
    
}