const preorderTraversal = function(root) {
    const result = [];
    function pushRoot(node){
        if (node !== null) {
            result.push(node.val);
            if (node.left !== null){ 
                pushRoot(node.left);
            }
            if (node.right !== null) {
                pushRoot(node.right);
            } 
        }
    }
    pushRoot(root);
    return result;
};