高精度组合数学

import java.io.*; import java.util.*; import java.math.*; public class Main {
    public static void main(String[] args) throws FileNotFoundException     {
            Scanner cin = new Scanner(new BufferedInputStream(System.in)); //Scanner cin = new Scanner(new FileInputStream("t.txt"));         int n = cin.nextInt(); while (n != 0)         {
                String st = cin.next();             BigInteger a = new BigInteger("1");             BigInteger ans = new BigInteger("0"); int[] f = new int[30]; for (int i = 0; i < n; i++)                 f[i] = st.charAt(i) - 'A'; for (int i = n - 1; i >= 0; i--)             {
    int x = 0; for (int j = 0; j < i; j++) if (f[j] < f[i])                         x++;                 f[i] -= x;             } for (int i = n - 1; i >= 0; i--)             {
    int x = f[i];                 ans = ans.add(BigInteger.valueOf(x).multiply(a));                 a = a.multiply(BigInteger.valueOf(26 - i));             }             System.out.println(ans);             n = cin.nextInt();         }     } }