206. Reverse Linked List

难度: Easy

题目描述

Given the head of a singly linked list, reverse the list, and return the reversed list.

 

Example 1:

Input: head = [1,2,3,4,5]
Output: [5,4,3,2,1]

Example 2:

Input: head = [1,2]
Output: [2,1]

Example 3:

Input: head = []
Output: []

 

Constraints:

• The number of nodes in the list is the range [0, 5000].
• -5000 <= Node.val <= 5000

 

Follow up: A linked list can be reversed either iteratively or recursively. Could you implement both?

解题思路

代码实现

解决方案

java

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

        ListNode currentListNode = head;
        ListNode insertListNode = head.next;
        currentListNode.next = null;
        while (insertListNode != null) {
            ListNode insert = insertListNode;
            insertListNode = insertListNode.next;
            insert.next = currentListNode;
            currentListNode = insert;
        }

        return currentListNode;

    }
}