lintcode: Reverse Linked List

Reverse a linked list.

For linked list 1->2->3, the reversed linked list is 3->2->1

Reverse it in-place and in one-pass

最先应该想到的是用栈先依次存储起来，然后弹栈。然而空间复杂度不满足要求。

其实，先保存第二个指针的下一个指针，再翻转两个指针之间的指向就可以达到原地翻转的目的。

参考：​​http://www.thinksaas.cn/group/topic/395495/​​

/**
 * 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||head->next==NULL){
            return head;
        }

        ListNode *p1=head;
        ListNode *p2=head->next;

        while(p2){
            ListNode *p2next=p2->next;

            if(p1==head){
                p1->next=NULL;
            }

            p2->next=p1;
            p1=p2;
            p2=p2next;
        }


        //return p2;
        return p1;
    }
};