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