linked_list_cycle_2.py

class Solution:
    def detectCycle(self, head: Optional[ListNode]) -> Optional[ListNode]:
        slow, fast = head, head
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next
            if slow == fast:
                break
        else:
            return None
        slow = head
        while slow != fast:
            slow = slow.next
            fast = fast.next
        return