input: Write a function to implement a simple HashMap class with put and get methods.
  assert h.get('key1') == 'value1'
  assert h.get('key2') == 'value2'
  assert h.get('key3') == None
output: class HashMap:
      def __init__(self):
          self.map = {}
      
      def put(self, key, value):
          self.map[key] = value
      
      def get(self, key):
          return self.map.get(key)
  
  # Example usage
  h = HashMap()
  h.put('key1', 'value1')
  h.put('key2', 'value2')