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function uniqueLetterString(s: string): number {    const index = new Map<string, number[]>();
    for (const [i, c] of Array.prototype.entries.call(s)) {        const arr = index.get(c) ?? [-1];        arr.push(i);        index.set(c, arr);    }    let res = 0;
    for (const arr of index.values()) {        arr.push(s.length);
        for (let i = 1; i < arr.length - 1; i++) {            res += (arr[i] - arr[i - 1]) * (arr[i + 1] - arr[i]);        }    }    return res;}export default uniqueLetterString;