Reverse Linked List.cpp

/*
Reverse Linked List
===================

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

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

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

Example 3:
Input: head = []
Output: [] 

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

Follow up: A linked list can be reversed either iteratively or recursively. Could you implement both?
*/

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */

class Solution
{
public:
  ListNode *reverseList(ListNode *head)
  {
    if (!head || !head->next)
      return head;
    ListNode *dummy = NULL;
    while (head)
    {
      auto next = head->next;
      head->next = dummy;
      dummy = head;
      if (!next)
        break;
      head = next;
    }
    return head;
  }
};