input: Write a function to design a data structure in Python to store integer values. The data structure should have methods to add a value to the list and to find the minimum value in the list. If the list is empty, the find_min method should return None.
  class IntList:
      def __init__(self):
          self.values = []
      
      def add_value(self, value):
          self.values.append(value)
      
      def find_min(self):
          return min(self.values) if self.values else None
  
  assert IntList().find_min() is None
  int_list = IntList()
  int_list.add_value(3)
  int_list.add_value(4)
  int_list.add_value(5)
  assert int_list.find_min() == 3
  int_list.add_value(1)
  assert int_list.find_min() == 1
output: 
  def __init__(self):
      self.values = []
  
  def add_value(self, value):
      self.values.append(value)
  
  def find_min(self):
      return min(self.values) if self.values else None
  