leetcode-license-key-formatting.js

var licenseKeyFormatting = function(S, K) {

    let str = S.replace(/-/g, "").toUpperCase() //去除"-"並改成大寫
    let result = '' //每次迴圈跑完就對這個空字串添加字符
    let countK = 0 
    for(let i =str.length-1; i >= 0; i--) { //由後往前遍歷字串，起始點是最後一個字，終點是第0個字
        letter = str[i]
        result += letter 
        countK ++
        if(countK == K){ //如果剛好加到第 K 個，就要再加上"-"
                result += '-'
                countK = 0 
        }
    }
    if(result != 0 && result[result.length - 1] == '-') {
        result = result.substring(0, result.length-1)
    }
    return result.split("").reverse().join("")

};