/**

 * 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 head;

        ListNode* preHead = head;

        ListNode* current = preHead->next;

        ListNode* curNext;

        head->next = NULL;

        while(current){

            curNext = current->next;

            current->next = preHead;

            preHead = current;

            current = curNext;

        }

        return preHead;

    }

};