【简单】环形链表

让快指针的速度是慢指针的两倍，这样当慢指针走k步，快指针一定是2k步，两个指针此时一定相遇

class Solution {
public:
    bool hasCycle(ListNode *head) {
        ListNode *slow = head, *fast = head;
        while (fast != NULL && fast->next != NULL)
        {
            slow = slow->next;
            fast = fast->next->next;
            if (slow == fast)
                return true;
        }
        return false;
    }
};