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;
    }
};