1234567891011121314
export default function singleNumber(nums: number[]): number {    const once = new Set<number>();    const twice = new Set<number>();    for (const n of nums) {        if (once.has(n)) {            twice.add(n);        } else {            once.add(n);        }    }    twice.forEach((n) => once.delete(n));    return [...once.keys()][0];}