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