经典的农夫养牛问题

一个农夫养了一头牛，三年后，这头牛每年会生出1头牛，生出来的牛三年后，又可以每年生出一头牛……问农夫10年后有多少头牛?n年呢？

1.

public class Cow {
    static int count = 1;
    private static void feedCow(int year,int age){
        year++;
        age++;
        if(year<=30){
            if(age>=3){
                count++;
                feedCow(year,0);
            }
            feedCow(year,age);
        }
    }

    public static void main(String[] args) {
        new Cow().feedCow(0, 0);
        System.out.println(count);
    }
}

2.

public class Cow {
    public static int count = 0;
    public Cow(int year){
        count++;
        for(int i=3+year;i<=10;i++){
            new Cow(i);
        }
    }

    public static void main(String[] args) {
        new Cow(0);
        System.out.println(count);
    }
}

3.

 

public class Cow {
    private int age;
    public Cow(){
        age = 0;
    }
    public Cow play(){
        age ++;
        return age > 3 ? new Cow():null;
    }
   
    public static void main(String[] args) {
        List<Cow> list = new ArrayList<Cow>();
        list.add(new Cow());
        for(int i = 0; i <= 10;i++){
            for(int j =0;j<list.size();j++){
                Cow cow = list.get(j).play();
                if(cow != null)
                    list.add(cow);
            }
        }
        System.out.println("10年后，共有：" + list.size());
    }
}