detectCycle.py

def detectCycle(self, head):
    slow, fast = head, head
    while fast and fast.next:
        slow = slow.next
        fast = fast.next.next
        if slow == fast: break
    
    if not fast or not fast.next: return None
    slow = head
    while slow != fast:
        slow = slow.next
        fast = fast.next
    return slow