求一个简单的算法，急，先谢了

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        while(n<=0){
            System.out.println("n must greater than zero, please input a number again");
            n = in.nextInt();
        }
        int a[][] = new int[n][n];
        for(int i = 0; i<n;i++){
            for(int j=0;j<n;j++){
                if(j>=i)
                    a[i][j]=i;
                else
                    a[i][j]=j;
                System.out.print(a[i][j]+" ");
            }
            System.out.println();
        }   
    }
}