451.根据字符串频率排序

/**
 * @param {string} s
 * @return {string}
 */
var frequencySort = function (s) {
    let map = new Map();
    for (let i of s) {
        map.set(i, (map.get(i) || 0) + 1);
    }
    const list = [...map.keys()];
    list.sort((a, b) => map.get(b) - map.get(a));
    const sb = [];
    const size = list.length;
    for (let i = 0; i < size; i++) {
        const ch = list[i];
        for (let j = 0; j < map.get(ch); j++) {
            sb.push(ch);
        }
    }
    return sb.join('');
};