168. Excel Sheet Column Title

excel表列名称，给定整数 columnNumber，返回在 Excel表中相对应的列名称。

• 时间复杂度 O(N)，空间复杂度O(1)
• Runtime: 68 ms, faster than 82.33%
• Memory Usage: 38.9 MB, less than 9.05%

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