task 8

void fill_right_brother(TreeNode* root) {
    // root - самый левый на уровне,
    // у всех вершин уровня корректно заполнено поле right_brother
    for(;;) {
        TreeNode* left_node = nullptr;
        TreeNode* cur_node = nullptr;

        for (TreeNode* node = root; ;) {

            if (node->left) {
                if (cur_node)
                    cur_node->right_brother = node->left;
                cur_node = node->left;
                if (!left_node) left_node = cur_node;
            }

            if (node->right) {
                if (cur_node)
                    cur_node->right_brother = node->right;
                cur_node = node->right;
                if (!left_node) left_node = cur_node;
            }

            if (!node->right_brother)
                break;
            node = node->right_brother;
        }

        if (!left_node)
            break;
        root = left_node;
    }
}