在Java中将*打印为三角形？

*
**
***
****


   *
  **
 ***
****


  *
 ***
*****

到目前为止,我能够做左对齐的三角形,但我似乎无法得到另外两个.我试过谷歌搜索但没有出现.有人可以帮忙吗？到目前为止我有这个.

import java.util.*;
public class Prog673A
{
    public static void leftTriangle()
    {
        Scanner input = new Scanner (System.in);
        System.out.print("How many rows: ");
        int rows = input.nextInt();
        for (int x = 1; x <= rows; x++)
        {
            for (int i = 1; i <= x; i++)
            {
                System.out.print("*");
            }
            System.out.println("");
        }
    }
    public static void rightTriangle()
    {
        Scanner input = new Scanner (System.in);
        System.out.print("How many rows: ");
        int rows = input.nextInt();
        for (int x = 1; x <= rows; x++)
        {
            for (int i = 1; i >= x; i--)
            {
                System.out.print(" ");
            }
            System.out.println("*");
        }
    }
    public static void centerTriangle()
    {

    }
    public static void main (String args [])
    {
        Scanner input = new Scanner (System.in);
        System.out.println("Types of Triangles");
        System.out.println("t1. Left");
        System.out.println("t2. Right");
        System.out.println("t3. Center");
        System.out.print("Enter a number: ");
        int menu = input.nextInt();
        if (menu == 1)
            leftTriangle();
        if (menu == 2)
            rightTriangle();
        if (menu == 3)
            centerTriangle();
    }
}

样本输出：

Types of Triangles
1.  Left
2.  Right
3.  Center
Enter a number (1-3):  3
How many rows?: 6

     *
    ***
   *****
  *******
 *********
***********