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;
        
    }
};