function alternate(s: string): number {
    // get a set of character in [s]
    let chars: string[] = Array.from(new Set<string>(s.split('')))

    // get everys pair of 2 character in [s]
    let pairs: [string, string, string?, number?][] = []
    for (let i = 0; i < chars.length; i++) {
        for (let j = i + 1; j < chars.length; j++) {
            pairs.push([chars[i], chars[j]])
        }
    }

    // check each pair, save info after remove other character except character in that pair
    for (let i = 0; i < pairs.length; i++) {
        let pair = pairs[i]
        let astr = s.split('').filter(c => pair.includes(c))

        pair.push(astr.join(''))
        pair.push(astr.every((c, j) => c != astr[j + 1]) ? astr.length : 0)
    }

    console.log(pairs)

    // return the largest valid result in [pairs]
    return Math.max(0, ...pairs.map(([a, b, str, result]) => result))
}