快慢指针真的很好用！

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool hasCycle(ListNode *head) {
    //快慢指针
    ListNode* fast = head;
    ListNode* slow = head;
    while(fast != nullptr && fast->next!= nullptr )
    {
        fast = fast->next->next;
        slow = slow->next;
        if(fast == slow)
        {
            return true;
        }
    }
    return false;
    }
};