How can I make this faster, without making it multi-threaded?

Question

My program is working fine, in terms of output, but for some of my test cases it takes too long to find an answer (sometimes taking 18 seconds). I would like to know how I can improve the performance of my code.

What my code does: It's a take on Pebble Solitaire. The user inputs n number of games and after that inputs a strings of length 23 that contains a combinations of only 'o' (pebble) and '-' (empty space). If there are 2 adjacent pebbles and an empty space on either side, ie (oo- OR -oo), then you remove the middle pebble and you swap other two pieces with each other, ex 'oo-' will turn into '--o'.

My current approach is pretty much an exhaustive approach where it tries out every possible move and results the move set with the least number of pebbles left.

I would like to know how I can improve this solution without making it multi-threaded.

Here is what I have:

package Pebble;

import java.util.Scanner;

public class PebbleSolitaire {

    public static void main(String[] args){

        Scanner input = new Scanner(System.in);
        int numOfGames = Integer.parseInt(input.nextLine());

        while (numOfGames > 0){

            char[] values = input.nextLine().toCharArray();
            long startTime = System.nanoTime();
            System.out.println(solve(values));
            System.out.println("Time to finish in ms: " + (System.nanoTime() - startTime) / 1000000);
            numOfGames--;
        }
        input.close();
    }

    private static int solve(char[] game){

        if(game != null && game.length == 0){
            return -1;
        }

        int result = 0;

        for (int i = 0; i < game.length; i++){
            if(game[i] == 'o'){
                result++;
            }
        }

        //print(game);

        for (int i = 0; i < game.length; i++ ){

            char[] temp = new char[game.length];
            copyArray(temp, game);

            if (i-2 >= 0 && temp[i] == '-' && temp[i-2] == 'o' && temp[i-1] == 'o'){//move pebble forwards
                temp[i-1] = temp[i-2] = '-'; 
                temp[i] = 'o';
                result = Math.min(result, solve(temp));
            }

            copyArray(temp, game);

            if(i+2 < temp.length && temp[i] == '-' && temp[i+1] == 'o' && temp[i+2] == 'o'){//move pebble backwards
                temp[i+1] = temp[i+2] = '-';
                temp[i] = 'o';
                result = Math.min(result, solve(temp));
            }
        }
        return result;
    }

    private static void copyArray(char[] copy, char[] og){
        for(int x = 0; x < copy.length; x++){
            copy[x] = og[x];
        }
    }
    private static void print(char[] c){
        for(char ch: c){
            System.out.print(ch);
        }
        System.out.println();
    }
}

My sample input and output:

2
-o----ooo----o----ooo--
6
Time to finish in ms: 0
oooooooooo-ooooooooooo-
4
Time to finish in ms: 18149

EDIT: Would making this completely iterative drastically improve the performance?

Answer 1

Maybe you can improve this parte:

  for (int i = 0; i < game.length; i++ ){

        char[] temp = new char[game.length];
        copyArray(temp, game);

        if (i-2 >= 0 && temp[i] == '-' && temp[i-2] == 'o' && temp[i-1] == 'o'){//move pebble forwards
            temp[i-1] = temp[i-2] = '-'; 
            temp[i] = 'o';
            result = Math.min(result, solve(temp));
        }

        copyArray(temp, game);

        if(i+2 < temp.length && temp[i] == '-' && temp[i+1] == 'o' && temp[i+2] == 'o'){//move pebble backwards
            temp[i+1] = temp[i+2] = '-';
            temp[i] = 'o';
            result = Math.min(result, solve(temp));
        }
    }

to:

    for (int i = 0; i < game.length; i++ ){
        char[] temp = null;

        if (i-2 >= 0 && game[i] == '-' && game[i-2] == 'o' && game[i-1] == 'o'){//move pebble forwards
            temp = new char[game.length];
            copyArray(temp, game);
            temp[i-1] = temp[i-2] = '-'; 
            temp[i] = 'o';
            result = Math.min(result, solve(temp));
        }



        if(i+2 < game.length && game[i] == '-' && game[i+1] == 'o' && game[i+2] == 'o'){//move pebble backwards

            if(temp == null) temp = new char[game.length];             

            copyArray(temp, game);
            temp[i+1] = temp[i+2] = '-';
            temp[i] = 'o';
            result = Math.min(result, solve(temp));
        }
    }

Basically, only creating and "copyArray(temp, game);" when strictly necessary.