剑指Offer(牛客网) 反转链表

题目描述
输入一个链表，反转链表后，输出新链表的表头。

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

    ListNode(int val) {
        this.val = val;
    }
}*/
1.双指针
public class Solution {
    public ListNode ReverseList(ListNode head) {
        if(head==null) return null;
            ListNode pre = null;//保存head的前一个结点
            ListNode next = null;//保存head的后一个结点
           while(head!=null){
              next = head.next;
               head.next = pre;
               pre = head;
               head = next;
           }
        return pre;
    }
}

2.递归

class Solution {
    public ListNode reverseList(ListNode head) {
            if(head == null||head.next == null){
                return head;
            }
            ListNode cur = reverseList(head.next);
            head.next.next = head;
            head.next = null;
            return cur;
    }
}