206. Reverse Linked List*

206. Reverse Linked List*

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

Title 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?

C ++ implementation 1

Flip the list is very classic problem, iterative approach needs to heart.

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;
    }
};

2 in C ++

Recursive method.

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;
    }
};