     },
     "outputs": [],
     "source": [
      "def unique_chars(string):\n",
      "    return len(set(string)) == len(string)"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
 
</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",
    "    if string is None:\n",
    "        return False\n",
     },
     "outputs": [],
     "source": [
      "def unique_chars(string):\n",
    "    if string is None:\n",
    "        return False\n",
      "    return len(set(string)) == len(string)"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
keep keep 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