字串数_hdu_1261(大数极致).java

字串数

Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 65536/32768 K (Java/Others)
Total Submission(s): 2661    Accepted Submission(s): 606

Problem Description

一个A和两个B一共可以组成三种字符串:"ABB","BAB","BBA".

给定若干字母和它们相应的个数,计算一共可以组成多少个不同的字符串.

 

Input

每组测试数据分两行,第一行为n(1<=n<=26),表示不同字母的个数,第二行为n个数A1,A2,...,An(1<=Ai<=12),表示每种字母的个数.测试数据以n=0为结束.

 

Output

对于每一组测试数据,输出一个m,表示一共有多少种字符串.

 

Sample Input

2
1 2
3
2 2 2
0

 

Sample Output

3
90

 

Source

浙江工业大学第四届大学生程序设计竞赛

 

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/*
 * (a1+a2+...)!/(a1!*a2!*...)
 */
import java.io.InputStreamReader;
import java.math.BigInteger;
import java.util.Scanner;

public class Main {//把大数用到极致了
	public static void main(String[] args) {
		Scanner input=new Scanner(new InputStreamReader(System.in));
		int n;
		while((n=input.nextInt())!=0){
			int a[]=new int[n];
			long sum=0;
			for(int i=0;i<n;i++){
				a[i]=input.nextInt();
				sum+=a[i];
			}
			BigInteger a1=NUM(sum);
			BigInteger s=BigInteger.ONE;
			for(int i=0;i<n;i++){
				s=s.multiply(NUM(a[i]));
			}
		//	System.out.println(a1+"   "+s);
			System.out.println(a1.divide(s));
		}
	}

	private static BigInteger NUM(long sum) {//计算阶乘
		BigInteger sum1=BigInteger.ONE;
		for(int i=2;i<=sum;i++)
			sum1=sum1.multiply(BigInteger.valueOf(i));
		return sum1;
	}
}