问题描述

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

测试样例

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

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

说明

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

解题

思路

记录 pre, curr 指针, 在遍历时记录每轮 next 指针，逆向即可。

补充：

  1. 链表
  2. 时间复杂度 O(n)

代码

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/**
 * 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) {
        ListNode pre = null, curr = head;
        while(curr != null) {
            ListNode next = curr.next;
            
            // 后移一位
            curr.next = pre;
            pre = curr;
            curr = next;
            
        }
        
        return pre;
    }
}