input: Write a function to implement a data structure that stores integer values and can find the minimum value among them. The data structure should support adding a value and finding the minimum value.
  class IntList:
      def __init__(self):
          self.values = []
     
      def add_value(self, value):
          self.values.append(value)
      
      def find_min(self):
          return min(self.values)
  
  assert IntList().add_value(5).find_min()==5
  assert IntList().add_value(3).add_value(1).find_min()==1
  assert IntList().add_value(4).add_value(2).add_value(6).find_min()==2
  assert IntList().add_value(8).add_value(7).add_value(9).find_min()==7
  assert IntList().find_min()==None
output: def add_value(self, value):
      self.values.append(value)
      return self
      
  def find_min(self):
      return min(self.values) if self.values else None