Create a triangle out of stars using only recursion

Question

I need to to write a method that is called like printTriangle(5);. We need to create an iterative method and a recursive method (without ANY iteration). The output needs to look like this:

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

This code works with the iterative but I can't adapt it to be recursive.

public void printTriangle (int count) {
    int line = 1;
    while(line <= count) {
        for(int x = 1; x <= line; x++) {
            System.out.print("*");
        }
        System.out.print("\n");
        line++;
    }
}

I should note that you cannot use any class level variables or any external methods.

Answer 1

Example in python (just for the sake of prototyping, but I hope the idea gets through):

#!/usr/bin/env python

def printTriangle(n):
    if n > 1:
        printTriangle(n - 1)
    # now that we reached 1, we can start printing out the stars 
    # as we climb out the stack ...
    print '*' * n

if __name__ == '__main__':
    printTriangle(5)

Output looks like this:

$ python 2717111.py
*
**
***
****
*****

Answer 2

Notice in your iterative approach that you have two counters: the first is what line you are on line, and the second is what position on the line you are on x. You could create a recursive function that takes two parameters and uses them as nested counters, y and x. Where you decrement x until it reaches 0, then decrement y and set x = y, until both x and y are 0.

You could also notice that each successive line in the triangle is the previous line plus one star. If your recursive function returns a string of stars for the previous line, the next line is always that string plus one more star. So, your code would be something like:

public String printTriangle (int count) {
    if( count <= 0 ) return "";

    String p = printTriangle(count - 1);
    p = p + "*";
    System.out.println(p);

    return p;
 }