this.levelOrder = function(root) {
    if (!root) {
        return;
    }

    const queue = [];
    queue.push(root);

    while (queue.length > 0) {
        const node = queue.shift();
        process.stdout.write(node.data + " ");

        if (node.left) {
            queue.push(node.left);
        }
        if (node.right) {
            queue.push(node.right);
        }
    }
};