Thread: N queens problem! Please help !I don't really know what the f... is going on ?

I need to solve the N queens problem (place n queens on a chess board size nxn so that the queens don't attack each other). Consist of 4 class :

Queen.java : hold the row and column of the queen

Stack.java : a data structure stack to hold the queen.

Nqueens.java : to solve the problem

testN.java : to test the solution

the error I got is <identifier> expected in the testN.java, I don't really know what is going on, everything seems normal so far.

Here are the codes:

public class Queen{
 
  int row;
  int column;
 
  public Queen(int r,int c){
  row=r;
  column=c;
  } 
 
  public int getRow(){
  return row;
  }
 
  public int getCol(){
  return column;
  }
 
  public boolean isConflict(Queen q)
  {
  boolean ans= false;
  //row and collum conflict
  if(row==q.row||column==q.column) ans=true;
  //same diagonal if the vector creats by two queens has same absolute value for x and y coordinator.
  else if(Math.abs(row-q.row)==Math.abs(column-q.column)) 
    ans=true;
  return ans;
  }
 
  public String toString(){
  String ans="";
  ans=ans+"( "+row+" , "+column+" )";
  return ans;
  }
 
}

public class Stack
{
private Queen[] data;
private int count;
 
public  Stack(int n){
  data= new Queen[n];
 }
 
public void push(Queen item)
{
  if(count==data.length) increaseSize();
data[count]=item;
count++;
}
 
public int getSize(){
return count;
}
 
public Queen pop(){
Queen ans= null;
if(count !=0)ans =data[count-1];
count--;
return ans;
}
 
public Queen lookitem(int i){
Queen ans=null;
if(count !=0)ans =data[i];
return ans;
}
 
public Queen peek()
{
Queen ans= null;
if(count !=0)ans =data[count-1];
return ans;
}
 
public void increaseSize(){
Queen[] temp=new Queen[2*data.length+1]; 
 
 
for(int i=0;i<data.length;i++)
{
temp[i]=data[i];
}
 
data=temp;
}
 
 
}

public class Nqueens{
 
private Stack s;
private int N;
 
public Nqueens(int size)
{
s=new Stack(size); 
N=size;  
}
 
//Solve the nqueens problem
public void solution()
{
 
s.push(new Queen(1,1));s.push(new Queen(2,1));
 
boolean solve=false;
 
while(s.getSize()!=0&&solve==false)
{
boolean conflict=false;int i=0;
while(i<s.getSize()-1&&conflict==false)
{
conflict=s.peek().isConflict(s.lookitem(i));  
i++;      
}
if (conflict=false)
{
while(s.getSize()!=0&&s.peek().getRow()==N)
{
s.pop();
}
if(s.getSize()!=0)
{
Queen qq=new Queen(s.peek().getRow(),s.peek().getCol()+1);  
s.pop();
s.push(qq);
}
}
else if(s.peek().getRow()==N) solve=true;
else s.push(new Queen(s.peek().getRow()+1,1));
}
 
}
 
public String toString(){
  String ans="";
    for(int i=0;i<s.getSize();i++){
    ans=ans+s.lookitem(i).toString()+"  ";
 
}
return ans;
}}

public class testN{
  public static void main(String arg[]){
 
    Queen q1=new Queen(1,1);
    Queen q2=new Queen(2,2);
    Queen q3=new Queen(2,1);
    Queen q4=new Queen(4,3);
    System.out.println(q1);
    System.out.println(q2);
    System.out.println(q1.isConflict(q4));
     System.out.println(q4.isConflict(q3));
  Stack s=new Stack(0);
  s.push(q1);s.push(q3);
  s.push(new Queen(3,6));
  System.out.println("Element on s "+s.peek());
  } 
 
  Nqueens qq = new Nqueens(4);
qq.solution();
 
  System.out.println(qq);  
 
}

Originally Posted by helloworld922

You have some code in your testN class which isn't inside of any method (last 3 lines).

Sorry I don't know what you mean ?

I created a object qq which belongs to the Nqueens class. Then use object qq to activate the method solution ( method solution in the Nqueens class).

Then I want to print out the result using toString() method.

ok i fixed it , thanks a lot. i know what you mean now.