Printing *s as triangles in Java?

Question

My assignment in my Java course is to make 3 triangles. One left aligned, one right aligned, and one centered. I have to make a menu for what type of triangle and then input how many rows is wanted. The triangles have to look like this

*
**
***
****


   *
  **
 ***
****


  *
 ***
*****

So far I was able to do the left aligned triangle but I can't seem to get the other two. I tried googling but nothing turned up. Can anyone help? I have this so far.

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();
    }
}

Sample Output:

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

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

Answer 1

Hint: For each row, you need to first print some spaces and then print some stars. The number of spaces should decrease by one per row, while the number of stars should increase.

For the centered output, increase the number of stars by two for each row.

Answer 2

Ilmari Karonen has good advice, and I'd just like to generalize it a bit. In general, before you ask "how can I get a computer to do this?" ask "how would I do this?"

So, if someone gave you an empty Word document and asked you to create the triangles, how would you go about doing it? Whatever solution you come up with, it's usually not hard to translate it to Java (or any other programming language). It might not be the best solution, but (hopefully!) it'll work, and it may point you to a better solution.

So for instance, maybe you would say that you'd type out the base, then go up a line, then type the next highest line, etc. That suggests that you can do the same in Java -- create a list of Strings, base-to-top, and then reverse them. That might suggest that you can just create them in reverse order, and then not have to reverse them. And then that might suggest that you don't need the list anymore, since you'll just be creating and printing them out in the same order -- at which point you've come up with essentially Ilmari Karonen's advice.

Or, maybe you'd come up with another way of doing it -- maybe you'd come up with Ilmari Karonen's idea more directly. Regardless, it should help you solve this and many other problems.