#eclipse自动补全

#倒立正三角形
public class test {
  public static void main(String[] args) {
    int i,j;
    for (i = 0; i <=4; i++) {
      for ( j = 0; j <= 4 - i; j++) {
        System.out.print("*");
      }
      System.out.println();
    }
  }

}

#数组大小排序
import java.util.Arrays;

public class arrtest {
  public static void main(String[] args) {
    System.out.println("请输入数组个数：");
    int c = getint();
    
    int[] arr = new int[c];
    for (int i = 0; i < c; i++) {
      System.out.println("请输入第"+(i+1)+"整数");
      arr[i]=getint();
    }
    System.out.println("排序前："+Arrays.toString(arr));
    paixu(arr);
    
  }

  private static void paixu(int[] arr) {
    int b;
    for (int i = 0; i < arr.length; i++) {
      for (int j = i+1; j < arr.length; j++) {
        if (arr[i] < arr[j]) {
          b = arr[i];
          arr[i] = arr[j];
          arr[j] = b;
        }
      }
    }
    System.out.println("排序后："+Arrays.toString(arr));
    
  }
  private static int getint() {
    Scanner sc = new Scanner(System.in);
    return sc.nextInt();
  }

}

#水仙花数：是指一个三位数，其各位数字立方和等于该数本身
package test;

import java.util.Scanner;

public class shuixianhua {
  public static void main(String[] args) {
    for (int i = 100; i < 1000; i++) {
      int q = i % 10;
      int w = (i / 10) % 10;
      int e = i /100;
      if (i == (q*q*q)+(w*w*w)+(e*e*e)) {
        System.out.println(i);
      }
    }
  }
}

求s=a+aa+aaa+aaaa+aa...a

package test;

import java.util.Scanner;

public class testtest {
  public static void main(String[] args) {
    System.out.println("请输入相加次数:");
    int a =getint();
    System.out.println("请输入数值:");
    int b =getint1();
    int first =b;
    int s = 0;
    for (int i = 0; i < a; i++) {
            s += b;
            b = b * 10 + first;
        }
    
     System.out.println("相加后的值为："+s);
    
  }
  private static int getint1() {
    Scanner sc = new Scanner(System.in);
    return sc.nextInt();
  }

  private static int getint() {
    Scanner sc = new Scanner(System.in);
    return sc.nextInt();
  }
}

#青蛙跳一只青蛙一次可以跳1级或2级台阶，N级的台阶总共有多少种跳法
分析：
/**
1层：1种
2层： 2种
3层:   3种
4层:    5种
5层:      8种
6层:       13种
7层          21种
总结规律
2层=1层+1层
3层=1层+2层
4层=2层+3层
5层=3层+4层
6层=4层+5层
7层=5层+6层
**/
第一种写法
 public static void main(String[] args){
          System.out.println("请输入台阶数：");
          try (Scanner input = new Scanner(System.in)) {
              int n=input.nextInt();
                            //frogjump方法
              long result = frogjump(n);
        // long result = (int)Math.pow(n-3, 2)+1;
              System.out.println("共有"+result+"种方法");
          }
      }
//frogjump计算方法
    static int frogjump(int n ){
        if(n==1){
            return 1;
        }else if(n==2){
            return 2;
        }else{
            int a = 1;
            int b = 2;
            int tmp = 0;
            for(int i=3;i<=n;i++){
                tmp = a+b;
                a = b;
                b = tmp;
            }
            return tmp;
        }
    }

第二种写法
public static void main(String[] args){
          System.out.println("请输入台阶数：");
          try (Scanner input = new Scanner(System.in)) {
              int n=input.nextInt();
                            //调用jump方法
              long result = jump(n);
              System.out.println("共有"+result+"种方法");
          }
      }
//jump计算方法
   public static int jump(int n){
          if(n==0){
              return 0;
          }
          else{
          int[] frog=new int[3];
          frog[0]=1;
          frog[1]=2;
          for(int i=2;i<n;i++){
              frog[i%3]=frog[(i-1)%3]+frog[(i-2)%3];
          }
      //返回跳的次数
          return frog[(n-1)%3];
      }
}
第N总解法---，掌握数学规律后，想咋写就咋写

#这天是一年的第几天？
public static void main(String[] args){
        System.out.println("----这一天是这一年的第几天-----");
       //设置日期格式
      SimpleDateFormat simpleDate = new SimpleDateFormat("yyyy/MM");
      Scanner sc =new Scanner(System.in);
      System.out.println("请输入年份");
      String dyear=sc.nextLine(); 
      System.out.println("请输入月份");
      String dmouth=sc.nextLine();
      System.out.println("请输入日");
      int dayint =sc.nextInt();
      Calendar rightNow = Calendar.getInstance();
      int daysum=0;
      try{
        int a =Integer.parseInt(dmouth);
        for (int i = 1; i < a; i++) {
          rightNow.setTime(simpleDate.parse(dyear+"/"+i));
          int days=0;
          days =rightNow.getActualMaximum(Calendar.DAY_OF_MONTH);
          System.out.print(i+"月:"+days+"天"+" ");
          daysum +=days;
      }
      }catch(ParseException e){e.printStackTrace();}
      System.out.println("\n"+"年第："+(daysum+dayint)+"天");
       //getActualMaximum获取天数最大值
      int days = rightNow.getActualMaximum(Calendar.DAY_OF_MONTH);//根据年月 获取月份天数
      System.out.println(days);
  }