     "outputs": [],
     "source": [
      "def unique_chars_inplace(string):\n",
      "    for char in string:\n",
      "        if string.count(char) > 1:\n",
      "            return False\n",
      "    return True"
     ]
 
</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_inplace(string):\n",
    "    if string is None:\n",
    "        return False\n",
      "    for char in string:\n",
      "        if string.count(char) > 1:\n",
      "            return False\n",
      "    return True"
     ]
keep keep add 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