from structures import build_tree

def valid_bst(root):
    def valid(node, low, high):
        if node is None:
            return True
        if low is not None and node.val <= low:
            return False
        if high is not None and node.val >= high:
            return False
        return (valid(node.left, low, node.val)
                and valid(node.right, node.val, high))
    return valid(root, None, None)