20210124.md

Algorithm

206. Reverse Linked List

Description

Reverse a singly linked list.

Example:

Input: 1->2->3->4->5->NULL
Output: 5->4->3->2->1->NULL

Follow up:

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

Solution

/**
 * 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){
        // 如果节点为空或者只有一个元素直接返回即可
        if(head == null || head.next == null){
            return head;
        }
        // 定义前节点
        ListNode pre = null;
        // 定义当前节点
        ListNode now = head;
        // 如果当前节点不为空，继续遍历
        while(now!=null){
            // 先定义临时变量保存下一个节点
            ListNode next = now.next;
            // 当前节点的下一个节点指向pre，这样实现了当前节点的反转
            now.next = pre;
            // 同时当前节点指向now，表示后移一位
            pre = now;
            // now节点指向next，同样后移一位
            now = next;
        }
        return pre;
    }
}

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