      "## Test Cases\n",
      "\n",
      "* '' -> True\n",
      "* 'foo' -> False\n",
      "* 'bar' -> True"
     ]
    },
    {
 
</s> remove     "* Can you assume the string is ASCII?\n",
 </s> add     "* Can we assume the string is ASCII?\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",
    "* None -> False\n",
      "## Test Cases\n",
      "\n",
    "* None -> False\n",
      "* '' -> True\n",
      "* 'foo' -> False\n",
      "* 'bar' -> True"
     ]
    },
    {
keep add keep 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