Xác thực BST

def is_valid_bst_opt(root):
    if isinstance(root, list):
        r = build_tree(root)
        return is_valid_bst_opt_helper(r)
    return is_valid_bst_opt_helper(root)

def is_valid_bst_opt_helper(root: TreeNode) -> bool:
    def validate(node, low, high):
        if not node:
            return True
        if not (low < node.val < high):
            return False
        return validate(node.left, low, node.val) and validate(node.right, node.val, high)
    return validate(root, float('-inf'), float('inf'))

def is_valid_bst_brute(root):
    if isinstance(root, list):
        r = build_tree(root)
        return is_valid_bst_brute_helper(r)
    return is_valid_bst_brute_helper(root)

def is_valid_bst_brute_helper(root: TreeNode) -> bool:
    vals = []
    def inorder(node):
        if not node: return
        inorder(node.left)
        vals.append(node.val)
        inorder(node.right)
    inorder(root)
    for i in range(len(vals) - 1):
        if vals[i] >= vals[i + 1]:
            return False
    return True