code: public static String indexToColName(int index) {
  		if (index < 0) {
  			return null;
  		}
  		final StringBuilder colName = StrUtil.builder();
  		do {
  			if (colName.length() > 0) {
  				index--;
  			}
  			int remainder = index % 26;
  			colName.append((char) (remainder + 'A'));
  			index = (int) ((index - remainder) / 26);
  		} while (index > 0);
  		return colName.reverse().toString();
  	}
docs: 将Sheet列号变为列名
  
  @param index 列号, 从0开始
  @return 0->A; 1->B...26->AA
  @since 4.1.20