单链表反序。

这是一道常见的笔试题，leetcode上有两道题。Reverse Linked List和Reverse Linked List II。

Reverse Linked List

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if(head==NULL)  {return head;}
        ListNode *p1,*p2,*p3;

        p1 = head;
        p2 = head->next;
        while (p2!=NULL)
        {
            p3 = p2->next;
            p2->next = p1;
            p1 = p2;
            p2 = p3;
        }
        head->next = NULL;
        head = p1;
        return p1;
    }
};

Reverse Linked List II 反序从m至n区间中的链表。

class Solution {
public:
    ListNode* reverseBetween(ListNode* head, int m, int n) {
        int index = 1;
        ListNode *temp = head;
        ListNode *temp2 = temp;
        bool is = false;
        while (index<m) {
            is = true;
            temp2 = temp;
            temp = temp->next;
            index++;
        }
        ListNode *p1, *p2, *p3;

        p1 = temp;
        p2 = temp->next;

        while (p2 != NULL&&index<n) {
            p3 = p2->next;
            p2->next = p1;
            p1 = p2;
            p2 = p3;
            index++;
        }
        temp->next = p2;
        if (is) {
            temp2->next = p1;
            return head;
        }
        else {
            return p1;
        }
    }
};