Gomoku game Question (given at hackerrank.com)

[Ashish A Bansal]

Question is at this site :Hacker Rank Question
The problem is when i run this code on that site ,it shows that("Invalid move. You are trying to place the stone on an occupied cell.
JudgeBot won the game"). Please run it on above given site and check my code.

import java.util.*; /*imported class for taking input output from user*/
import java.math.*;

public class Solution {

    public static void main(String[] args) {
        Scanner stdin = new Scanner(System.in); 
        String Playertype = stdin.next();
        String waste = stdin.next();
        String movenumber =stdin.next();
        if(movenumber.contains("1")){                  // fixed the first three moves of the game and used .contains because it was showing an error on " if (movenumber=="1").......
        	System.out.print("H:8");
        }
        else if (movenumber.contains("2")){
        	System.out.print("G:9");
        }
        else if (movenumber.contains("3")){
        	System.out.print("H:5");
        }
        else{
        String board[] = new String[15];
        for(int i =0;i<15;i++){
         board[i]=stdin.next();
        }
        char [][]target;                                   
        target = new char[board.length][];
        for(int i=0;i<board.length;i++)
        { target[i] = board[i].toCharArray();}    
        while(mainthing(target,Playertype)==1){}; // used for printing output and determining what the output should be.If the method returns 1 , it will again run that method and if it returns 0 ,the program is terminated. 
    }
    }
    
    public static  int mainthing(char target [][],String Playertype){
    	
    	int x,y;
        x=(int)(14*Math.random()+1);
        y=(int)(14*Math.random()+1);
        if(target [x][y]!='B' && target [x][y]!='W'){
        	char var= (char) (y+65);
            
        
	if(Playertype=="Black"){target[x][y]='B';}
        	else{target[x][y]='W';}
            if(var=='H'&&x==8){return 1;}
            if(var=='G'&&x==9){return 1;}
            if(var=='H'&&x==5){return 1;}
        	System.out.printf("%c:%d",var,x);
        return 0;	
        }
        else{
        	return 1;
        }
    
    }

}