将Excel列字母转换为数字的算法是什么？

public static int ExcelColumnNameTonumber(string columnName){    if (string.IsNullOrEmpty(columnName)) throw new ArgumentNullException("columnName");    columnName = columnName.ToUpperInvariant();    int sum = 0;    for (int i = 0; i < columnName.Length; i++)    {        sum *= 26;        sum += (columnName[i] - 'A' + 1);    }    return sum;}