typedef struct listNode {
    int val;
    struct listNode *next;
} listNode;

struct ListNode *detectCycle(struct ListNode *head) {
    if (head == NULL || head->next == NULL || head->next->next == NULL) return NULL;
    struct listNode *fast = head, *slow = head, *ret;
    while (fast->next != NULL && fast->next->next != NULL) {
        slow = slow->next;
        fast = fast->next->next;
        if (fast == slow) break;
    }
    if (fast->next == NULL || fast->next->next == NULL) return NULL;
    ret = head;
    while (slow != ret) {
        ret = ret->next;
        slow = slow->next;
    }
    return ret;
}