451.根据字符出现频率排序

451.根据字符出现频率排序

to tree 
/**
 * @param {string} s
 * @return {string}
 */
var frequencySort = function(s) {
    if(!s)return
    let map=new Map();
    for(let i of s){
        map.set(i,(map.get(i)||0)+1)
    }
    let arr=[...map.entries()].sort((v1,v2)=>{return v2[1]-v1[1]})
    let result=''
    for(let [k,v] of arr){
       result+=k.repeat(v)
    }
    return result
};