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
  
  # Example:
  # mf.add(3)
  # mf.add(1)
  # mf.add(2)
  # mf.find_median() should return 2.0
  
output:     
          self._data.append(num)
   
      def find_median(self):
          data = sorted(self._data)
          n = len(data)
          return (data[(n-1)//2] + data[n//2])/2
  