6. Reverse Linked List 逆转单链表

逆转单链表，比较简单，不细讲，扫描依次改变指针指向。

class Solution {public:    ListNode* reverseList(ListNode* head) {        if(head==nullptr)return head;        ListNode * tmp = head->next;        ListNode *prenode = head;        while(tmp!=NULL)        {              ListNode *nextnode = tmp->next;            tmp->next = prenode;            prenode = tmp;            tmp = nextnode;         }         head->next = NULL;         head = prenode;         return head;    }};