TheAlgorithms-C/leetcode/src/142.c

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