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import { TreeNode } from "../binary-tree-inorder-traversal/TreeNode.ts";
export default function preorderTraversal(root: TreeNode | null): number[] {    if (!root) return [];    const res: number[] = [];    preorder(root, (a) => res.push(a));    return res;}function preorder(root: TreeNode | null, output: (a: number) => void) {    if (!root) return;    output(root.val);    preorder(root.left, output);
    preorder(root.right, output);}