code: def put(self, key, value):
          
          self._cache[key] = value
          self._order.push(key)
          self._size += 1
docs: >>> c = LRUCache()
          >>> c.put(1, 'one')
          >>> c.get(1)
          'one'
          >>> c.size()
          1
          >>> c.put(2, 'two')
          >>> c.put(3, 'three')
          >>> c.put(4, 'four')
          >>> c.put(5, 'five')
          >>> c.get(5)
          'five'
          >>> c.size()
          5