3155589_ac_141ms_2344k.java

北大大牛代码 1240道题的原代码 超级权威

import java.util.*;
import java.math.*;

public class Main
{
	
	public static void main(String [] args)
	{
		new Main().run();
	}
	
	private void run()
	{
		Scanner cin = new Scanner (System.in);
		int n;
		String num;
		
		while(true)
		{
			n = cin.nextInt();
			if(n==0)
			{
				break;
			}
			num = cin.next();
			System.out.println(calc(num,"ABCDEFGHIJKLMNOPQRSTUVWXYZ"));
		}
	}

	private BigInteger calc(String num,String al)
	{
		if(num.length()==1)
		{
			return BigInteger.valueOf(al.indexOf(num.charAt(0)));
		}
		
		int index = al.indexOf(num.charAt(0));
		
		if(index==0)
		{
			return calc(num.substring(1),al.substring(1));
		}
		BigInteger ret = BigInteger.valueOf(index);
		
		ret = ret.multiply(A(al.length()-1,num.length()-1));
		
		return ret.add(calc(num.substring(1),al.substring(0,index)+al.substring(index+1)));
	}

	private BigInteger A(int n, int k) 
	{
		BigInteger ret = BigInteger.ONE;
		
		for(int i = n; i > n-k; i--)
		{
			ret = ret.multiply(BigInteger.valueOf(i));
		}
		return ret;
	}
	
}