'''
            fast = a + (b + c)k + b
            slow = a + b
            2a + 2b = a + kb + kc + b
                  a = kb + kc - b - c + c
                  a = (k - 1)(b + c) + c
'''
class Solution(object):


    def entryNodeOfLoop(self, head):
        slow, fast = head, head
        while fast and fast.next:
            slow, fast = slow.next, fast.next.next

            if slow == fast: # 第一次相遇
                slow = head  
                while slow != fast:
                    slow, fast = slow.next, fast.next
                return slow

        return None