task99.md

/*
    for example:
    [1, 15, 17, 10, 25]
    group like:
        1: {
            less: [15, 17], (1 < 5, 1 < 7)
            eq: [1],
            more: [10] (1 > 0)
        }
        2: {
            less: [25]
        }
    and group again every less and more group untill group size > 1: ...less, ...eq, ...more
*/

function group(arr, index = 0) {
    if (arr.length < 2) {
        return arr[0] || "";
    }

    let res = "";

    const groups = Array.from({length: 10}, _ => ({
        more: [],
        less: [],
        eq: [],
        active: false
    }));

    for (let i=0; i<arr.length; i++) {
        const char = parseInt(arr[i][index], 10);
        groups[char].active = true;
        if (arr[i].length - 1 === index) {
            groups[char].eq.push(arr[i]);
        } else {
            if (char < arr[i][index+1]) {
                groups[char].less.push(arr[i]);
            } else {
                groups[char].more.push(arr[i]);
            }
        }
    }
    for (let i=9; i>=0; i--) {
        const g = groups[i];
        if (g.active) {
            res += group(g.less, index + 1) + g.eq.join("") + group(g.more, index + 1);
        }
    }
    return res;
}

console.log(group([ "123", "124", "56", "90"]));
console.log(group(["12", "14", "15", "19", "2", "25", "21", "67"]));
console.log(group(["123", "124", "56", "90", "95", "87", "8", "88", "89"]));