input: Write a function to design a data structure that stores integer values and provides methods to add values and find the minimum value among them. If the list is empty, the function 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(5)
  int_list.add_value(3)
  int_list.add_value(10)
  assert int_list.find_min()==3
output: def find_min(self):
      return min(self.values) if self.values else None