   "metadata": {
    "kernelspec": {
     "display_name": "Python 2",
     "language": "python",
     "name": "python2"
    },
    "language_info": {
     "codemirror_mode": {
      "name": "ipython",
      "version": 2
 
</s>
   "name": "python3"
   "metadata": {
    "kernelspec": {
     "display_name": "Python 2",
     "language": "python",
   "name": "python3"
    },
    "language_info": {
     "codemirror_mode": {
      "name": "ipython",
      "version": 2
keep keep keep keep replace keep keep keep keep keep
 Polish unique chars challenge and solution

Add None input test case.  Update constraints.
arrays_strings/unique_chars/unique_chars_solution.ipynb