206. Reverse Linked List

/**
 * 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) {

        //for empty or single node
        if(head == NULL || head -> next == NULL)
            return head;

        ListNode* prev = NULL;
        ListNode* curr = head;
        ListNode* forward = NULL; 

        while(curr != NULL){
            forward = curr -> next;
            curr -> next = prev;
            prev = curr;
            curr = forward;

        }

        return prev;

    }
};