/*** Definition for singly-linked list.* struct ListNode {*     int val;*     ListNode *next;*     ListNode(int x) : val(x), next(NULL) {}* };*/
class Solution {
public:ListNode *detectCycle(ListNode *head) {set<ListNode *>obj;///为了防止元素内容相同，这里选择存储整个链表的节点while(head){if(obj.count(head)==1){return head;}obj.insert(head);head=head->next;}return NULL; }
};