剑指offer_反转链表

输入一个链表，反转链表后，输出新链表的表头。

方法一、直接反转

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

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    public ListNode ReverseList(ListNode head) {
        if(head==null) return null;
        ListNode m=null;
        ListNode cur=head;
        ListNode next=cur.next;
        while(next!=null){
            cur.next=m;
            m=cur;
            cur=next;
            next=next.next; 
        }
        cur.next=m;
        return cur;
        
    }
}

方法二：使用头插法

 1 /*
 2 public class ListNode {
 3     int val;
 4     ListNode next = null;
 5 
 6     ListNode(int val) {
 7         this.val = val;
 8     }
 9 }*/
10 public class Solution {
11     public ListNode ReverseList(ListNode head) {
12         ListNode newList=new ListNode(-1);
13         
14         while(head!=null){
15             ListNode next=head.next;
16             head.next=newList.next;
17             newList.next=head;
18             head=next;
19         }
20         return newList.next;
21     }
22 }

方法三：

递归

 1 /*
 2 public class ListNode {
 3     int val;
 4     ListNode next = null;
 5 
 6     ListNode(int val) {
 7         this.val = val;
 8     }
 9 }*/
10 public class Solution {
11     public ListNode ReverseList(ListNode head) {
12         if(head==null) return null;
13         if(head.next==null) return head;
14        ListNode next=head.next;
15        head.next=null;
16        ListNode newList= ReverseList(next);
17        next.next=head;
18        return newList;
19        }
20 }

 

posted @ 2019-08-26 10:12  chyblogs  阅读(109)  评论(0)    收藏  举报