text: '''
  Descripttion: 
  version: 
  Author: HuSharp
  Date: 2021-02-22 12:12:48
  LastEditors: HuSharp
  LastEditTime: 2021-02-23 23:16:08
  @Email: 8211180515@csu.edu.cn
  '''
  class A: 
      z = -1
      def f(self, x):
          return B(x-1)
  
  class B(A):
      n = 4
      def __init__(self, y):
          if y:
              self.z = self.f(y)
          else:
              self.z = C(y+1)
  
  
  class C(B):
      def f(self, x):
          return x