Lintcode 翻转链表

翻转一个链表

样例

给出一个链表1->2->3->null，这个翻转后的链表为3->2->1->null

分析：

/**
 * Definition of ListNode
 *
 * class ListNode {
 * public:
 *     int val;
 *     ListNode *next;
 *
 *     ListNode(int val) {
 *         this->val = val;
 *         this->next = NULL;
 *     }
 * }
 */
class Solution {
public:
    /**
     * @param head: The first node of linked list.
     * @return: The new head of reversed linked list.
     */
    ListNode *reverse(ListNode *head) {
        // write your code here
         if(head == NULL)
            return 0;
        ListNode *p = head;
        ListNode *P1 = NULL;
        ListNode *P2 = NULL;
        while(p != NULL)
        {
           P1 = p->next;
            p->next = P2;
            P2 = p;
            p = P1;
        }
        return P2;

    }
};