剑指Offer(牛客网) 反转链表

Posted 2023-01-23

来源：
​​​ https://www.nowcoder.com/practice/75e878df47f24fdc9dc3e400ec6058ca?tpId=13&tqId=11168&rp=1&ru=/ta/coding-interviews&qru=/ta/coding-interviews/question-ranking​​​

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;