/**

 * Definition for singly-linked list.

 * struct ListNode {

 *     int val;

 *     ListNode *next;

 *     ListNode(int x) : val(x), next(NULL) {}

 * };

 */

class Solution {

public:

    ListNode* reverseList(ListNode* head)

    {

        if(!head)

            return NULL;

        ListNode* a = NULL;

        ListNode* b = head;

        ListNode* c = b->next;

        while(c)

        {

            b->next = a;

            a = b;

            b = c;

            c = c->next;

        }

        b->next = a;

        return b;

    }

};