Python代码检查给定的二叉树是否为BST时出现问题

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def is_bst(root):
    return is_bst_helper(root, float('-inf'), float('inf'))

def is_bst_helper(node, min_val, max_val):
    if node is None:
        return True
    
    if node.val <= min_val or node.val >= max_val:
        return False
    
    return (is_bst_helper(node.left, min_val, node.val) and
            is_bst_helper(node.right, node.val, max_val))

# 示例用法
# 创建一个二叉树
root = TreeNode(4)
root.left = TreeNode(2)
root.right = TreeNode(6)
root.left.left = TreeNode(1)
root.left.right = TreeNode(3)
root.right.left = TreeNode(5)
root.right.right = TreeNode(7)

# 检查二叉树是否为BST
if is_bst(root):
    print("给定的二叉树是BST")
else:
    print("给定的二叉树不是BST")