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
  assert MedianFinder().add(1).add(3).add(2).find_median() == 2
  assert MedianFinder().add(5).add(3).add(4).find_median() == 4
  assert MedianFinder().add(7).add(2).add(9).add(4).find_median() == (4 + 7) / 2
  assert MedianFinder().add(10).add(6).add(8).find_median() == 8
  assert MedianFinder().add(1).add(2).find_median() == (1 + 2) / 2
output: 
  class MedianFinder:
   def __init__(self):
       self._data = []
   
   def add(self, num):
       self._data.append(num)
       return self
   
   def find_median(self):
       data = sorted(self._data)
       n = len(data)
       return (data[(n-1)//2] + data[n//2])/2
  