仅使用递归打印金字塔

Question

I managed to print a pyramid with loops like this:

void printtree() {
  for (int i=0; i<row; i++){
    for (int j=0; j<row-i-1; j++)
      System.out.print(" ");
    for (int k=row; k>row-i-1; k--)
      System.out.print("* ");
    System.out.println();
  }
}

The output looks like this:

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

I would like to make this using recursion only, how should I construct the function? My concern is with the nested loop, I could only interprete the single inside or ouside loop. I know it is imterpreted with (row-1) spaces followed by row # of "* " on each line, then change to a new line. What is the base case here?

Answer 1

Since you wanted solution to use recursion only. I assume you don't want any loops in the program. If you want to convert your program quickly into recursive approach, look at each for loop and think how you can convert it.

You can convert

 for (int j=0; j<row-i-1; j++)
      System.out.print(" ");

to function

static void printSpace(int j, int i) {
        if (j < row - i - 1) {
            System.out.print(" ");
            printSpace(j + 1, i);
        }
    }

Notice that above recursive function does same as your for loop. It requires same parameters which you have used in your for loop. Now this trick you can apply to for loop which prints star as well.

static void printStar(int k, int i) {
        if (k > row - i - 1) {
            System.out.print("* ");
            printStar(k - 1, i);
        }
    }

The similar trick can be applied to outer for loop and you can transform the code.

Complete code

static int row = 5;

    static void printtree() {
        printPyramid(0);
    }

    static void printPyramid(int i) {
        printSpace(0, i);
        printStar(row, i);
        System.out.println();

        if (++i < row)
            printPyramid(i);
    }

    static void printSpace(int j, int i) {
        if (j < row - i - 1) {
            System.out.print(" ");
            printSpace(j + 1, i);
        }
    }

    static void printStar(int k, int i) {
        if (k > row - i - 1) {
            System.out.print("* ");
            printStar(k - 1, i);
        }
    } 

Answer 2

static void recursion(int row, int k, int j)
{
if(row>10)
{
    return;
}
else
{
    if(k==row)
    {
    System.out.println();
    recursion(++row, 0,0);
    }
    else if(10-j>row)
    {
    System.out.print(" ");
    ++j;
    recursion(row, k, j);
    }
    else
    {

    System.out.print("* ");
    recursion(row, ++k, j);
    }   
}
}

call method

recursion(0,0,0);