下面程序是一个二叉排序树的，横线处应该填入的是（ ）。
class BinarySortTree:
def __init__(self):
self.root = None
def insert(self, key, value):
node = TreeNode(key, value)
if self.root is None:
self.root = node
return
current = self.root
while True:
if key < current.key:
___________________________
current = current.left
else:
if current.right is None:
current.right = node
return
current = current.right
def search(self, key):
current = self.root
while current:
if current.key == key:
return current.value
elif current.key > key:
current = current.left
else:
current = current.right
return None
def inorder_traversal(self, node):
if node:
self.inorder_traversal(node.left)
print(node.key, node.value)
self.inorder_traversal(node.right)
if current.left is None:
current.right = node
return
if current.right is None:
current.right = node
return
if current.right is None:
current.left = node
return
if current.left is None:
current.left = node
return