206. Reverse Linked List*

206. Reverse Linked List*

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

题目描述

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?

C++ 实现 1

翻转链表是非常经典的题, 迭代的方法需要铭记于心.

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if (!head) return nullptr;
        ListNode *prev = nullptr;
        while (head) {
            auto tmp = head->next;
            head->next = prev;
            prev = head;
            head = tmp;
        }
        return prev;
    }
};

C++ 实现 2

递归的方法.

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if (!head || !head->next) return head;
        auto tmp = head->next;
        auto new_head = reverseList(head->next);
        tmp->next = head;
        head->next = nullptr;
        return new_head;
    }
};