剑指offer_15:反转链表

定义一个函数，输入一个链表的头节点，反转该链表并输出反转后链表的头节点。

示例:
输入: 1->2->3->4->5->NULL
输出: 5->4->3->2->1->NULL

限制：
0 <= 节点个数 <= 5000

1、双指针

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        if(head==null) return null;
        ListNode cur=null;
        ListNode pre=head;
        while(pre!=null){
            ListNode node=pre.next;
            pre.next=cur;
            cur=pre;
            pre=node;
        }
        return cur;
    }
}

2、另一种双指针

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        if(head==null) return null;
        ListNode cur=head;
        while(head.next!=null){
            ListNode t=head.next.next;
            head.next.next=cur;
            cur=head.next;
            head.next=t;
        }
        return cur;
    }
}

3、递归

https://leetcode-cn.com/problems/fan-zhuan-lian-biao-lcof/solution/fan-zhuan-lian-biao-yi-dong-de-shuang-zhi-zhen-jia/

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        if(head==null||head.next==null) return head;
        ListNode newHead=reverseList(head.next);
        head.next.next=head;
        head.next=null;
        return newHead;
    }
}