input: Write a python function to maintain a list of the last n elements.
  assert LastNElements(3).push(1) is None
  assert LastNElements(3).push(2) is None
  assert LastNElements(3).push(3) is None
  assert LastNElements(3).get_elements() == [1, 2, 3]
  assert LastNElements(3).push(4) is None
  assert LastNElements(3).get_elements() == [2, 3, 4]
  assert LastNElements(3).push(5) is None
  assert LastNElements(3).get_elements() == [3, 4, 5]
output: class LastNElements:
      def __init__(self, n):
          self.n = n
          self.elements = []
  
      def push(self, element):
          if len(self.elements) == self.n:
              self.elements.pop(0)
          self.elements.append(element)
  
      def get_elements(self):
          return self.elements