(C#)计算字符串排列组合数 如"abcd"组合数为24 "aabb"组合数为6

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

namespace ConsoleApplication1
{
class Program
{
public static long GetA(int m, int n)
{
long num = -1;
if (m >= n && n >= 0 && m > 0)
{
num = 1;
for (int i = 0; i < n; i++)
{
num *= m - i;
}
return num;
}
return num;
}

public static long GetCombinationCount(string input)
{
long count;
if (input == null)
count = -1;
else if (input == string.Empty)
count = 0;
else
{
int length = input.Length;
if (length > 25)
count = -1;
else
{
Dictionary<char, int> dic = new Dictionary<char, int>();
foreach (char item in input)
{
if (!dic.ContainsKey(item))
dic.Add(item, 1);
else
dic[item]++;
}
count = GetA(length, length);
if (dic.Count != length)
{
Dictionary<char, int>.Enumerator enu = dic.GetEnumerator();
while (enu.MoveNext())
{
int tmp = enu.Current.Value;
if (tmp != 1)
count /= GetA(tmp, tmp);
}
}
}

}
return count;
}

static void Main(string[] args)
{
Console.WriteLine(GetA(50, 20));
Console.WriteLine(GetA(50, 20) * 10);
Console.WriteLine(GetCombinationCount("abcdefghijklmnopqrstuvwxy"));
Console.WriteLine(GetCombinationCount("abcdefghijklmnopqrstuvwxyz"));
Console.WriteLine(GetCombinationCount("1234567890"));
Console.WriteLine(GetCombinationCount("1122333456"));
Console.WriteLine(GetCombinationCount("1234567890") / GetA(2, 2) / GetA(2, 2) / GetA(3, 3));
}
}
}