Reverse Linked List

Reverse a singly linked list.

解析：

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