input: 
  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
  
  # >>> mf = MedianFinder()
  # >>> mf.add(5)
  # >>> mf.add(3)
  # >>> mf.add(7)
  # >>> mf.add(4)
  # >>> mf.find_median()
  # 4.5
  
  # >>> mf = MedianFinder()
  # >>> mf.add(1)
  # >>> mf.add(2)
  # >>> mf.find_median()
  # 1.5
  
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
  