from structures import build_tree

def is_balanced(root):
    def height_or_fail(node):
        if node is None:
            return 0
        left = height_or_fail(node.left)
        if left == -1:
            return -1
        right = height_or_fail(node.right)
        if right == -1 or abs(left - right) > 1:
            return -1
        return 1 + max(left, right)
    return height_or_fail(root) != -1