input: Write a Python function to parse a string representing an arithmetic expression into a tree structure.
  assert parse("2 + 3 * 4").data == '2'
  assert parse("2 + 3 * 4").left.data == '3'
  assert parse("2 + 3 * 4").left.left.data == '4'
  assert parse("2 + 3 * 4").left.left.right == None
output: class Node:
      def __init__(self, data):
          self.data = data
          self.left = None
          self.right = None
  
  def parse(expression):
      tokens = expression.split()
      
      def build_tree(tokens):
          if not tokens:
              return None
          node = Node(tokens.pop(0))
          if tokens:
              node.left = build_tree(tokens)
          if tokens:
              node.left.left = build_tree(tokens)
          return node
      
      return build_tree(tokens)