      "    * Return False\n",
      "    \n",
      "Complexity:\n",
      "* Time: O(n)\n",
      "* Space: Additional O(m), where m is the number of unique characters in the set"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
 
</s> remove     "* Space: In-place"
 </s> add     "* Space: O(1)" </s> remove     "* Space: Additional O(m), where m is the number of unique characters in the hash map"
 </s> add     "* Space: Additional O(n)"
    "* Space: Additional O(n)"
      "    * Return False\n",
      "    \n",
      "Complexity:\n",
      "* Time: O(n)\n",
    "* Space: Additional O(n)"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
keep keep keep keep replace keep keep keep keep keep
 Tweaked unique chars challenge algorithm discussion. 
arrays_strings/unique_chars/unique_chars_solution.ipynb