input: Write a function to design a data structure in Python to store integer values with methods to add values and 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()==None
  int_list = IntList()
  int_list.add_value(3)
  int_list.add_value(5)
  int_list.add_value(1)
  assert int_list.find_min()==1
  assert IntList().add_value(4).find_min()==4
  assert IntList().add_value(4).add_value(2).find_min()==2
  assert IntList().add_value(4).add_value(2).add_value(10).find_min()==2
output: 
  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
  