LeetCode 451. Sort Characters By Frequency

問題：數值由6和9組成，修改其中一個數字讓其可以獲得最大數值
解法：將數值轉字串陣列，判斷第一個找到的6將其修改為9，重新組合併轉為整數型態即可
JavaScript

/**
 * @param {number} num
 * @return {number}
 */
var maximum69Number = function (num) {
    if (num < 1 || num > Math.pow(10, 4)) {
        return;
    }
    let length = num.toString().length;
    let numArray = num.toString().split("");
    for (let i = 0; i < length; i++) {
        if (numArray[i] == '6') {
            numArray[i] = '9';
            break;
        }
    }

    return parseInt(numArray.join(""));
};