yunyin/Test.java

package me.yoqi;

import java.math.BigInteger;

public class Test {

	public static void main(String args[]) {
		int init = 25;
		double pk = 0;
		for (int n = 3; n < init; n++) {
			BigInteger nn = new BigInteger(String.valueOf(Math.pow(365, n)));
			long temp = (combination(365, n).multiply(factorial(n)).divide(nn)).longValue();
			pk = 1 - temp;
			if (pk >= 0.5) {
				System.out.println("最小K：" + n);
				break;
			}
		}
		System.out.print(factorial(25));
	}

	/**
	 * 计算阶乘数，即n! = n * (n-1) * ... * 2 * 1
	 * 
	 * @param n
	 * @return
	 */
	private static BigInteger factorial(int n) {
		BigInteger nn=new BigInteger(Integer.toString(n));
		return (BigInteger) ((n > 1) ? factorial(n - 1).multiply(nn) : 1);
	}

	/**
	 * 计算排列数，即A(n, m) = n!/(n-m)!
	 * 
	 * @param n
	 * @param m
	 * @return
	 */
	public static BigInteger arrangement(int n, int m) {
		return (BigInteger) ((n >= m) ? factorial(n).divide(factorial(n - m)) : 0);
	}

	/**
	 * 计算组合数，即C(n, m) = n!/((n-m)! * m!)
	 * 
	 * @param n
	 * @param m
	 * @return
	 */
	public static BigInteger combination(int n, int m) {
		return (BigInteger) ((n >= m) ? factorial(n).divide(factorial(n - m)).divide(factorial(m)) : 0);
	}
}