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import { inorder } from "../binary-tree-inorder-traversal/inorder.ts";import { TreeNode } from "../binary-tree-inorder-traversal/TreeNode.ts";
// }export default function treeToDoublyList(    root: TreeNode | null,): TreeNode | null {    if (!root) return null;    const tree = new TreeNode();    let first = true;    let current = tree;    inorder(root, (a) => {        if (first) {            tree.val = a;            first = false;        } else {            current.right = new TreeNode(a, current);            current = current.right;        }    });    tree.left = current;    current.right = tree;    return tree;}