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;
}