23. Excel数转换

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication21
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine(DecToExcel(26));
            Console.WriteLine(ExcelToDec("AB"));
        }

        static string DecToExcel(int n)
        {
            if (n <= 0)
            {
                throw new Exception("input must greater than zero");
            }
            StringBuilder sb = new StringBuilder();
            int a = 0;
            while (n > 0)
            {
                a = (n - 1) % 26;
                n = (n - 1) / 26;
                sb.Insert(0, ((char)(a + ‘A‘)));
            }

            return sb.ToString();
        }

        static int ExcelToDec(string input)
        {
            int result = 0;
            for (int i = 0; i < input.Length; i++)
            {
                result = result * 26 + input[i] - ‘A‘ + 1;
            }
            return result;
        }
    }
}