给定链表的头节点head，反转链表并且返回反转后的链表
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode() : val(0), next(nullptr) {}
* ListNode(int x) : val(x), next(nullptr) {}
* ListNode(int x, ListNode *next) : val(x), next(next) {}
* };
*/
class Solution {
public:
ListNode* reverseList(ListNode* head) {
ListNode* pre = nullptr;
ListNode* cur = head;
ListNode* next = head;
while(cur){
next = next->next;
cur->next = pre;
pre = cur;
cur = next;
}
return pre;
}
};
