input: 
  Write a class in Python that can find the median of an unsorted list of numbers.
  
  class MedianFinder:
   def __init__(self):
       self._data = []
   
   def add(self, num):
       self._data.append(num)
   
   def find_median(self):
       data = sorted(self._data)
       n = len(data)
       return (data[(n-1)//2] + data[n//2])/2
  
  median_finder = MedianFinder()
  median_finder.add(5)
  median_finder.add(3)
  median_finder.add(8)
  median_finder.add(4)
  median_finder.add(2)
  assert median_finder.find_median() == 4
  
  median_finder = MedianFinder()
  median_finder.add(12)
  median_finder.add(7)
  median_finder.add(5)
  median_finder.add(8)
  assert median_finder.find_median() == 7.5
  
  median_finder = MedianFinder()
  median_finder.add(9)
  median_finder.add(3)
  median_finder.add(6)
  assert median_finder.find_median() == 6
  
output:     
      def __init__(self):
          self._data = []
      
      def add(self, num):
          self._data.append(num)
      
      def find_median(self):
          data = sorted(self._data)
          n = len(data)
          return (data[(n-1)//2] + data[n//2])/2
  