168. Excel表列名称

/**
 * @param {number} columnNumber
 * @return {string}
 */
var convertToTitle = function(columnNumber) {
    let res = [];
    while(columnNumber > 0){
        columnNumber -= 1;
        res.unshift( String.fromCharCode(columnNumber % 26 + 65) );
        columnNumber = Math.floor( columnNumber / 26 );
    }
    return res.join(‘‘)
};