题目：

题解：

struct ListNode* detectCycle(struct ListNode* head) {struct ListNode *slow = head, *fast = head;while (fast != NULL) {slow = slow->next;if (fast->next == NULL) {return NULL;}fast = fast->next->next;if (fast == slow) {struct ListNode* ptr = head;while (ptr != slow) {ptr = ptr->next;slow = slow->next;}return ptr;}}return NULL;
}