package pa;
import java.util.*;
public class Yu{
    public static void main(String[] args){
    Scanner s=new Scanner(System.in);
    int a=s.nextInt();
    int i,j;
    for(i=1;i<=a;i++)
        for(j=1;j<2*a-1;j++)
            {
            if(j<a-i)
            {for(j=1;j<a-i;j++)
            System.out.print(" ");}
            if(j==a-i)
            {for(j=a-i;j<2*a-1;j++)
            System.out.print("*");}
            if(j>a-i)
            {for(j=a*a-2;j>a-i;j--)
            System.out.print(" ");}
            if(j==2*a-1)
            {System.out.print(" ");
            System.out.println();}
            }
            
    }
}

j-- 那里 j最后等于a-1 之后回到第二个for循环 j++ 又回到j--那个循环 死循环了

哦，谢谢！