https://leetcode.com/problems/single-number-iii

function singleNumber(nums: number[]): number[] {
    const seen = new Set<number>();
    const seenTwice = new Set<number>();
    for(const i of nums) {
        if (seen.has(i)) {
            seenTwice.add(i);
        } else {
            seen.add(i);
        }
    }

    return [...seen].filter(i => !seenTwice.has(i));
};