input: Write a python function to create a binary search tree and insert elements into it.
  assert insert(None, 10).key == 10
  assert insert(Node(10), 5).left.key == 5
  assert insert(Node(10), 15).right.key == 15
output: class Node:
      def __init__(self, key):
          self.key = key
          self.left = None
          self.right = None
  
  def insert(root, key):
      if root is None:
          return Node(key)
      else:
          if root.key < key:
              root.right = insert(root.right, key)
          else:
              root.left = insert(root.left, key)
      return root