public class QueenCounter {

    private IQueenState myBoard;
    private int mySize;
    private int myCount;
    public QueenCounter(int n){
        mySize = n;
        myBoard = new QBoard(n);
        solve(0);
        System.out.println("# places = "+myCount);
    }
    /**
     * Queens have been placed in all columns [0..col), try to place
     * a queen in column <code>col</code> and all columns after 
     * it, returning true if this is possible, false otherwise.
     * @param col is left-most column with no queen in it
     * @return true if a queen can be placed in all columns [col..size)
     */
    public void solve(int col){
        
        if (col == mySize) {
            myCount++;
            return;
        }
        
        // try each row until all are tried
        
        for(int r=0; r < mySize; r++){
            if (myBoard.safeToPlace(r,col)){
                myBoard.setQueen(r,col,true);
  
                solve(col+1);
                
                myBoard.setQueen(r,col,false);
            }
        }
    }
    
    public static void main(String[] args){
        int size = 11;
        QueenCounter q = new QueenCounter(size);
    }
}