public static void main(String[] args){
        NQueens test = new NQueens();
        List<String[]> result = test.solveNQueens(4);
        for(String[] strs: result){
            for(int i = 0; i < strs.length; i++){
                System.out.println(strs[i]);
            }
            System.out.println();
        }

    }