function postOrder(root) {
    let str = '';
    let current = root;
    let stack = [];
    
    while (current != null) {
        str = ' ' + current.data + str;
        if (current.left != null) {
            stack.push(current.left);
        }
        if (current.right != null) {
            stack.push(current.right);
        }
        current = stack.pop();
    }
    
    process.stdout.write(str.substring(1));
}