class Solution:
  def findCycleStart(self, head):
    slow, fast = head, head
    while fast and fast.next:
      slow = slow.next
      fast = fast.next.next
      // when the two pointers meet, rest anyone pointer back to head and then step one at a time
			// until they meet again. the point they meet is the starting of the cycle.
      if slow == fast:
        slow = head
        while slow!=fast:
          slow = slow.next
          fast = fast.next
        return slow
    return None