public List<String[]> solveNQueens(int n) {
        result = new ArrayList<String[]>();
        chessBoard = new char[n][n];
        len = n;
        for(int i = 0; i < n; i++){
            for(int j = 0; j < n; j++){
                chessBoard[i][j] = '.';
            }
        }
        dfs(0);
        return result;
    }