收集面试题（十四）（母牛问题）

// * 牛产仔问题。有一头母牛，它每年年初要生一头小母牛；每头小母牛从第四个年头起， 

//* 每年年初也要生一头小母牛。按此规律，若无牛死亡，第20年头上共有多少头母牛？ 

public class CowCount {
	public static void main(String[] args) {

		int year = 20;

		ArrayList<Cow> cows = new ArrayList<Cow>();
		cows.add(new Cow(3));

		for (int i = 0; i < year; i++) {
			ArrayList<Cow> newCows = new ArrayList<Cow>();
			for (Cow cow : cows) {
				cow.haveBirthday(newCows);
			}
			cows.addAll(newCows);
		}

		System.out.println(year + "年光阴似箭，共有牛" + cows.size() + "头。");

	}
}

class Cow {

	private int age = 0;

	public Cow(int age) {
		this.age = age;

	}

	public void haveBirthday(ArrayList<Cow> cows) {
		age++;

		if (age > 3) {
			cows.add(new Cow(0));
		}

	}
}