class BST_Node:
   def __init__(self, key):
      self.key = key
      self.left = None
      self.right = None
      self.parent = None

   def insert_elem(self, node):
      if self.key > node.key:
         if self.left is None:
            self.left = node
            node.parent = self
         else:
            self.left.insert_elem(node)
      elif self.key < node.key:
         if self.right is None:
            self.right = node
            node.parent = self
         else:
            self.right.insert_elem(node)

   def search_node(self, key):
      if self.key > key:
         if self.left is not None:
            return self.left.search_node(key)
         else:
            return None
      elif self.key < key:
         if self.right is not None:
            return self.right.search_node(key)
         else:
            return None
      return self

class BSTree:
   def __init__(self):
      self.root = None

   def add_elem(self, key):
      new_node = BST_Node(key)
      if self.root is None:
         self.root = new_node
      else:
         self.root.insert_elem(new_node)

   def search_node(self, key):
      if self.root is not None:
         return self.root.search_node(key)

   def get_smallest_elem(self):
      if self.root is not None:
         current = self.root
         while current.left is not None:
            current = current.left
         return current.key

   def get_largest_elem(self):
      if self.root is not None:
         current = self.root
         while current.right is not None:
            current = current.right
         return current.key

my_instance = BSTree()

print('菜单（假设没有重复的键）')
print('加')
print('最小值')
print('最大值')
print('退出')

while True:
   my_input = input('要执行哪个操作？').split()

   operation = my_input[0].strip().lower()
   if operation == 'add':
      key = int(my_input[1])
      my_instance.add_elem(key)
   if operation == 'smallest':
      smallest = my_instance.get_smallest_elem()
      print('最小元素是：{}'.format(smallest))
   if operation == 'largest':
      largest = my_instance.get_largest_elem()
      print('最大元素是：{}'.format(largest))
   elif operation == 'quit':
      break

菜单（假设没有重复的键）
加 <key>
最小值
最大值
退出
要执行哪个操作？添加5
要执行哪个操作？添加8
要执行哪个操作？添加11
要执行哪个操作？添加0
要执行哪个操作？添加3
要执行哪个操作？最小值
最小元素是：0
要执行哪个操作？最大值
最大元素是：11
要执行哪个操作？退出