Reverse a singly linked list

Reverse a singly linked list.

 /**
  * 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 == NULL){
             return head;
         }
         ListNode *p1,*p2,*p3;
         p1 = head;
         p2 = p1->next;
         if (p2 == NULL){
             return head;
         }
         p3 = p2->next;
         while (p2 != NULL){
             p2->next = p1;
             p1 = p2;
             p2 = p3;
             if (p3 != NULL){
                 p3 = p3->next;
             }
         }
         head->next = NULL;
         return p1;

     }
 };