206. Reverse Linked List.cpp

/*
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) {
        //Iterative
        ListNode *p=head, *q=NULL, *r=NULL;
        while(p){
            r = q;
            q = p;
            p = p->next;
            q->next = r;
        }
        head = q;
        return q;
        
        //Recursive
        if(head==NULL || head ->next==NULL)
          return head;

        ListNode *smallans = reverseList(head->next);
        ListNode *tail = head->next;
        tail->next = head;
        head ->next = NULL;

        return smallans;
    }
};