206.reverse-linked-list.cpp

/*
 * @lc app=leetcode id=206 lang=cpp
 *
 * [206] Reverse Linked List
 *
 * https://leetcode.com/problems/reverse-linked-list/description/
 *
 * algorithms
 * Easy (52.82%)
 * Total Accepted:    520.4K
 * Total Submissions: 985.2K
 * Testcase Example:  '[1,2,3,4,5]'
 *
 * 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?
 *
 */
#include <vector>
using namespace std;

struct ListNode
{
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};
class Solution
{
  public:
    ListNode *reverseList(ListNode *head)
    {
        if (!head || !head->next)
            return head;
        ListNode *rev = reverseList(head->next);
        head->next->next = head;
        head->next = nullptr;
        return rev;
    }
};