206. reverse linked list

class Solution {
public:
    ListNode* (ListNode* head) {
        if (head == NULL)
            return NULL;
            
        ListNode *s = NULL;
        ListNode *p = head;
        while (p != NULL)
        {
            ListNode *q = p->next;
            p->next = s;
            s = p;
            p = q;
        }
        return s;
    }
};