我不断收到这个错误，我不知道如何纠正它。我正在尝试在 BST 类中实现方法 count_less。我正在类 _BSTNode 中编写一个辅助方法，并使用间接递归在 count_less 中调用该辅助方法。
谢谢你。
class BST:
"""A Binary Search Tree."""
def __init__(self: 'BST', container: list =[]) -> None:
"""
Initialize this BST by inserting the items from container (default [])
one by one, in the order given.
"""
# Initialize empty tree.
self.root = None
# Insert every item from container.
for item in container:
self.insert(item)
def __str__(self: 'BST') -> str:
"""
Return a "sideways" representation of the values in this BST, with
right subtrees above nodes above left subtrees and each value preceded
by a number of TAB characters equal to its depth.
"""
return self.root._str("") if self.root else ""
def insert(self: 'BST', item: object) -> None:
"""
Insert item into this BST.
"""
if self.root:
self.root.insert(item)
else:
self.root = _BSTNode(item)
def count_less(self: 'BST', item: object) -> int:
"""
Return the number of items in this BST that are strictly less tham
item.
"""
return self.root._helper(self.root, item)
class _BSTNode:
"""A node in a BST."""
def __init__(self: '_BSTNode', item: object,
left: '_BSTNode' =None, right: '_BSTNode' =None) -> None:
"""
Initialize this node to store item and have children left and right.
"""
self.item, self.left, self.right = item, left, right
def _str(self: '_BSTNode', indent: str) -> str:
"""
Return a "sideways" representation of the values in the BST rooted at
this node, with right subtrees above nodes above left subtrees and each
value preceded by a number of TAB characters equal to its depth, plus
indent.
"""
return ((self.right._str(indent + '\t') if self.right else '') +
indent + str(self.item) + '\n' +
(self.left._str(indent + '\t') if self.left else ''))
def insert(self: '_BSTNode', item: object) -> None:
"""
Insert item into the BST rooted at this node.
"""
if item < self.item:
if self.left:
self.left.insert(item)
else:
self.left = _BSTNode(item)
elif item > self.item:
if self.right:
self.right.insert(item)
else:
self.right = _BSTNode(item)
# else: # item == self.item
# pass # nothing to do: item is already in the tree
def _helper(self, item):
count = 0
if not self.item:
return 0
if self.item < item:
count += 1
count += count_less(self.left, item)
count += count_less(self.right, item)
return count