代码实现：

快慢指针：

/*** Definition for singly-linked list.* struct ListNode {*     int val;*     struct ListNode *next;* };*/
bool hasCycle(struct ListNode *head) {// 快慢指针：快指针每次走两步，慢指针每次走一步，二者若重合则链表中有环，否则无环struct ListNode *slow = head, *fast = head;while (fast && fast->next) {fast = fast->next->next;slow = slow->next;if (fast == slow) {return true;}}return false;
}