Detecting cycle in a linked list

cycle-in-linked-list-floyd_alt.cpp

ListNode* detectCycle(ListNode* A) {
    if (A == NULL || A->next == NULL || A->next->next == NULL) return NULL;
    ListNode *slow = A, *fast = A;
    while (fast->next && fast->next->next) {
        slow = slow->next;
        fast = fast->next->next;
        if (slow == fast)
            break;
    }
    if (slow != fast) return NULL;
    
    int length = 1;
    for (fast = fast->next; fast != slow; fast = fast->next, length++);
    for (fast = A; length > 0; fast = fast->next, length--);
    for (slow = A; slow != fast; slow = slow->next, fast = fast->next);   
    
    return slow;
}