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
  
  """
  Find the median of an unsorted list of numbers using the MedianFinder class.
  
  Example:
  >>> mf = MedianFinder()
  >>> mf.add(2)
  >>> mf.add(1)
  >>> mf.add(3)
  >>> mf.find_median()
  2.0
  
  >>> mf = MedianFinder()
  >>> mf.add(6)
  >>> mf.add(15)
  >>> mf.add(4)
  >>> mf.add(2)
  >>> mf.add(7)
  >>> mf.add(1)
  >>> mf.find_median()
  4.5
  """
  
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
  