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(NULL == head || NULL == head->next) return head;
        ListNode *p1 = head, *p2 = head->next;
        p1->next = NULL;
        while(NULL != p2){
            ListNode* t = p2->next;
            p2->next = p1;
            p1 = p2;
            p2 = t;
        }
        return p1;
    }
};