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import { TreeNode } from "../binary-tree-inorder-traversal/TreeNode.ts";
export default function convertBST(root: TreeNode | null): TreeNode | null {    let pre = 0;    let cur: TreeNode | null | undefined = root;    const stack: (TreeNode | null | undefined)[] = [];    while (cur || stack.length !== 0) {        while (cur) {            stack.push(cur);            cur = cur.right;        }        cur = stack.pop();        if (cur) {            cur.val += pre;            pre = cur.val;            cur = cur?.left;        }    }    return root;}