【JAVA_SE】作业练习1029

1.有一对兔子，从出生后第3个月起每个月都生一对兔子，小兔子长到第三个月后每个月又生一对兔子，假如兔子都不死，问第二十个月的兔子对数为多少？(使用递归去解决)

public class Demo1 {
    public static void main(String[] args) {
        System.out.println(f(20));
        int sum=0;
        for(int i=1;i<=20;i++){
            sum+=f(i);
        }
        System.out.println(sum);
    }
    public static int  f(int n){
        if (n==1||n==2){
            return 1;
        }
        return f(n-1)+f(n-2);//递推公式
    }
}

2.定义一个数组,比如:int[] arr = {13,24,57,69,80}使用二分查找查找这个数组中的24元素对应的索引

public class Demo2 {
    public static void main(String[] args) {
        int[] arr = {13,24,57,69,80};
        System.out.println(getNumber(arr,24));//1
    }
    public static int getNumber(int [] arr,int target){
        int max=arr.length-1;
        int min=0;
        int mid=(max+min)/2;
        while(true){
            if(target>arr[mid]){
                min=mid+1;
            }else if(target<arr[mid]){
                max=mid-1;
            }else{
                return mid;
            }
            mid=(max+min)/2;
            if(min>max){
                return -1;
            }
        }
    }
}

3.统计大串中小串出现的次数
举例：在字符串” woaijavawozhenaijavawozhendeaijavawozhendehenaijavaxinbuxinwoaijavagun”中java出现了5次

public class Demo3 {
    public static void main(String[] args) {
        String str="woaijavawozhenaijavawozhendeaijavawozhendehenaijavaxinbuxinwoaijavagun";
        System.out.println("java出现了"+getNum(str,"java")+"次");
    }
    public static int getNum(String str,String target){
        int num=0;
        for(int i=0;i<str.length()-3;i++){
            String str2=str.substring(i, i+4);
            if("java".equals(str2)){
                num++;
            }
        }
        return num;
    }
}