Java递归打印星号

Question

I need to write a method that will print a star pattern that is specified below. The method's signature only passes in 1 parameter, and cannot be changed.

The method cannot have loops either, must simply call itself and solve the problem recursively. You can only use ONE method to solve this problem, not multiple.

public static void main(String[] args) {
    // Variables
    Scanner in = new Scanner(System.in); 
    Boolean go = true;
    int num;
    String answer;

    // Error catching structure 
    do {
        try {
            // Take input
            System.out.print("Enter a number > 1: ");
            num = in.nextInt();

            // Check to make sure num>1
            if (num <= 1) throw new Exception();

            // Call the method 
            System.out.println(printAsterisk(num));

            // Ask if the user wants to repeat
            System.out.print("Enter 'y' to repeat or 'n' to stop: ");
            answer = in.next().toLowerCase();

            // Check to see if we repeat
            if (answer.equals("n")) go = false; 
            else if (answer.equals("y")) go = true;
            else {
                System.out.println("Invalid input, program terminated.");
                break; // stops the program
            }
        }
        catch (InputMismatchException e) {
            System.out.println("Invalid input try again!");
            in.next(); // discards old token 
        }
        catch (Exception e) {
            System.out.println("Number is less than or equal to 1! Try again!");
        }

    }while(go); 
}

public static String printAsterisk(int n) {
    // Base case
    if (n == 0) return "";

    // Recursive Call
    String str = '*' + printAsterisk(n-1);
    System.out.println(str);

    return str;
}

The output required should look like this when calling printAsterisk(4):

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

However my method will print the following when called like this printAsterisk(4):

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

Answer 1

Your idea works well and can be tidied a bit. Having String s defined outside of the recursion is a bit yucky and the recursion implementation itself hides the symmetry of the solution a bit. Here is my variation on your solution:

    static void printAsterisk(int n) {
        printAsterisk(n, 1);
    }

    static void printAsterisk(int n, int m) {
        if (n < m) return;

        printStars(m);
        printAsterisk(n, m + 1);
        printStars(m);
    }

    static void printStars(int count) {
        char[] stars = new char[count];
        Arrays.fill(stars, '*');
        System.out.println(stars);
    }

    public static void main(String[] args) {
        printAsterisk(4);
    }

Answer 2

So I found the only solution to the problem that meets all of the question requirements. I changed the program a little bit and I added a global string variable. This allows me to manipulate the string and then reset it. Question was easy to solve by either editing the method signature and passing in more parameters, using loops, and or using multiple methods. This is the ONLY possible way to solve this problem with one method, one parameter being passed in (an integer), and NO loops. The following code will produce the right result, cheers...

import java.util.*;

public class RecursiveAsterisks {
    // Global Variables
    private static Scanner in = new Scanner(System.in); 
    private static Boolean go = true;
    private static int num;
    private static String answer;
    private static String s = "*";

    public static void main(String[] args) {    
        // Error catching structure 
        do {
            try {
                // Take input
                System.out.print("Enter a number > 1: ");
                num = in.nextInt();

                // Check to make sure num>1
                if (num <= 1) throw new Exception();

                // Call the method 
                printAsterisk(num);
                s = "*"; // reset string

                // Ask if the user wants to repeat
                System.out.print("Enter 'y' to repeat or 'n' to stop: ");
                answer = in.next().toLowerCase();

                // Check to see if we repeat
                if (answer.equals("n")) go = false; 
                else if (answer.equals("y")) go = true;
                else {
                    System.out.println("Invalid input, program terminated.");
                    break; // stops the program
                }
            }
            catch (InputMismatchException e) {
                System.out.println("Invalid input try again!");
                in.next(); // discards old token 
            }
            catch (Exception e) {
                System.out.println("Number is less than or equal to 1! Try 
again!");
            }
        }while(go);
    }

    // Recursive Method
    public static void printAsterisk(int n) {
        // Base case
        if (n == 0) return;

        // Recursive Call
        System.out.println(s);
        s += '*'; // concatenate string 
        printAsterisk(n-1);
        System.out.println(s.substring(n));
    }
}