     "metadata": {},
     "source": [
      "## Constraints\n",
      "\n",
      "* Can you assume the string is ASCII?\n",
      "    * Yes\n",
      "    * Note: Unicode strings could require special handling depending on your language\n",
      "* Can we assume this is case sensitive?\n",
      "    * Yes\n",
      "* Can you use additional data structures?  \n",
 
</s> remove     "* Can you assume the string is ASCII?\n",
 </s> add     "* Can we assume the string is ASCII?\n", </s> remove     "* Can you use additional data structures?  \n",
 </s> add     "* Can we use additional data structures?\n",
    "    * Yes\n",
    "* Can we assume this fits in memory?\n", </s> remove     "* Can you use additional data structures?\n",
 </s> add     "* Can we use additional data structures?\n",
    "    * Yes\n",
    "* Can we assume this fits in memory?\n",
    "* Can we assume the string is ASCII?\n",
     "metadata": {},
     "source": [
      "## Constraints\n",
      "\n",
    "* Can we assume the string is ASCII?\n",
      "    * Yes\n",
      "    * Note: Unicode strings could require special handling depending on your language\n",
      "* Can we assume this is case sensitive?\n",
      "    * Yes\n",
      "* Can you use additional data structures?  \n",
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