input: Write a Python function to check if a value exists in the BinarySearchTree.
  assert tree.contains(25) == True
  assert tree.contains(30) == False
  assert tree.contains(15) == True
output: class Node:
      def __init__(self, key):
          self.left = None
          self.right = None
          self.value = key
  
  class BinarySearchTree:
      def __init__(self):
          self.root = None
  
      def insert(self, key):
          if self.root is None:
              self.root = Node(key)
          else:
              self._insert(self.root, key)
  
      def _insert(self, root, key):
          if key < root.value:
              if root.left is None:
                  root.left = Node(key)
              else:
                  self._insert(root.left, key)
          else:
              if root.right is None:
                  root.right = Node(key)
              else:
                  self._insert(root.right, key)
  
      def contains(self, key):
          return self._contains(self.root, key)
  
      def _contains(self, root, key):
          if root is None:
              return False
          if root.value == key:
              return True
          elif key < root.value:
              return self._contains(root.left, key)
          else:
              return self._contains(root.right, key)