【数学水题】poj 1306 Combinations

Java处理大数还是很方便的，无压力AC。

import java.math.BigInteger;
import java.util.Scanner;


public class Main {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner cin=new Scanner(System.in);
		while(true){
			int a=cin.nextInt();
			int b=cin.nextInt();
			if(a==0 && b==0)
				return;
			BigInteger fir=factor(a);
			BigInteger sec=factor(a-b);
			BigInteger third=factor(b);
			BigInteger result=fir.divide(sec).divide(third);
			System.out.println(a+" things taken "+b+" at a time is "+result.toString()+" exactly.");
		}
	}
	
	public static BigInteger factor(int n){
		BigInteger goal=BigInteger.ONE;
		for(int i=n;i>=2;i--){
			goal=goal.multiply(new BigInteger(String.valueOf(i)) );
		}
		return goal;
	}

}