/**
knakul853
**/
class Solution {
public:
    ListNode* reverseList(ListNode* head) {

        ListNode* pre=NULL ;   ///1->2->3->4
        while(head)
        {
            ListNode* save_next = head->next;
            head->next=pre;
            pre = head;
            head = save_next;
        }
        return pre; //(o(n) and o(1))

    }
};