input: Write a python function to create a hash map that can add and retrieve key-value pairs.
  assert hmap.get('key1') == 'value1'
  assert hmap.get('key2') == 'value2'
  assert hmap.get('key3') == None
  hmap.add('key3', 'value3')
  assert hmap.get('key3') == 'value3'
output: class HashMap:
      def __init__(self):
          self.map = {}
  
      def add(self, key, value):
          self.map[key] = value
  
      def get(self, key):
          return self.map.get(key)
  
  hmap = HashMap()
  hmap.add('key1', 'value1')
  hmap.add('key2', 'value2')