input: Write a class to find the median of an unsorted list of numbers. The class should have a method to add numbers to the list and another method to return the median of the current list. If the list is empty, the find_median method should return None.
  
  class MedianFinder:
      def __init__(self):
          self._data = []
  
      def add(self, num):
          self._data.append(num)
  
      def find_median(self):
          if not self._data:
              return None
          data = sorted(self._data)
          n = len(data)
          return (data[(n-1)//2] + data[n//2])/2
  
  assert MedianFinder().find_median() == None
  median_finder = MedianFinder()
  median_finder.add(5)
  median_finder.add(3)
  assert median_finder.find_median() == 4.0
  median_finder = MedianFinder()
  median_finder.add(9)
  median_finder.add(2)
  median_finder.add(4)
  assert median_finder.find_median() == 4.0
output: 
  class MedianFinder:
      def __init__(self):
          self._data = []
  
      def add(self, num):
          self._data.append(num)
  
      def find_median(self):
          if not self._data:
              return None
          data = sorted(self._data)
          n = len(data)
          return (data[(n-1)//2] + data[n//2])/2
  