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function smallestSubsequence(s: string): string {    const ans: string[] = [" "];    const d: Set<string> = new Set();    Array.prototype.forEach.call(s, (c, i) => {        if (!d.has(c)) {            const t = s.slice(i);            for (                let a = ans[ans.length - 1];                a > c && t.indexOf(a) + 1;                a = ans[ans.length - 1]            ) {                const temp = ans[ans.length - 1];                d.delete(temp);                ans.pop();            }            ans.push(c);            d.add(c);        }    });    return ans.slice(1).join("");}export default smallestSubsequence;