206. 反转链表

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206. 反转链表

官方链接

https://leetcode-cn.com/problems/reverse-linked-list/

反转一个单链表。

示例:

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

进阶: 你可以迭代或递归地反转链表。你能否用两种方法解决这道题？

解法一 迭代

/**
 * 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 head;

        ListNode pre = null;
        ListNode prepre = null;
        while(head != null) {
            pre = head;
            head = head.next;
            pre.next = prepre;
            prepre = pre;
        }
        return pre;        
    }
}

class Solution:
    def reverseList(self, head: ListNode) -> ListNode:
        pre = None
        while head != None:
            head.next, pre, head = pre, head, head.next
        return pre

解法二 递归

/**
 * 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 p = reverseList(head.next);
        head.next.next = head;
        head.next = null;
        return p;
    }
}