      "   1          7 10  15\n",
      "               /\n",
      "              9\n",
      "\n",
      "In: 4  Out: 5\n",
      "In: 5  Out: 6\n",
      "In: 8  Out: 9\n",
      "In: 15 Out: None\n",
      "</pre>"
     ]
    },
    {
     "cell_type": "markdown",
 
</s> remove     "In: 4  Out: 5\n",
    "In: 5  Out: 6\n",
    "In: 8  Out: 9\n",
    "In: 15 Out: None\n",
 </s> add     "In: None  Out: Exception\n",
    "In: 4     Out: 5\n",
    "In: 5     Out: 6\n",
    "In: 8     Out: 9\n",
    "In: 15    Out: None\n", </s> remove     "            return\n",
 </s> add     "            # We reached the root, the original input node\n",
    "            # must be the largest element in the tree.\n",
    "            return None\n", </s> remove     "* Time: O(n)\n",
    "* Space: O(h), where h is the height of the tree"
 </s> add     "* Time: O(h), where h is the height of the tree\n",
    "* Space: O(h), where h is the recursion depth (tree height), or O(1) if using an iterative approach"
    "In: None  Out: Exception\n",
    "In: 4     Out: 5\n",
    "In: 5     Out: 6\n",
    "In: 8     Out: 9\n",
    "In: 15    Out: None\n",
      "   1          7 10  15\n",
      "               /\n",
      "              9\n",
      "\n",
    "In: None  Out: Exception\n",
    "In: 4     Out: 5\n",
    "In: 5     Out: 6\n",
    "In: 8     Out: 9\n",
    "In: 15    Out: None\n",
    "In: None  Out: Exception\n",
    "In: 4     Out: 5\n",
    "In: 5     Out: 6\n",
    "In: 8     Out: 9\n",
    "In: 15    Out: None\n",
    "In: None  Out: Exception\n",
    "In: 4     Out: 5\n",
    "In: 5     Out: 6\n",
    "In: 8     Out: 9\n",
    "In: 15    Out: None\n",
    "In: None  Out: Exception\n",
    "In: 4     Out: 5\n",
    "In: 5     Out: 6\n",
    "In: 8     Out: 9\n",
    "In: 15    Out: None\n",
      "</pre>"
     ]
    },
    {
     "cell_type": "markdown",
keep keep keep keep replace replace replace replace keep keep keep keep keep
 Polish bst successor challenge and solution (#82)

Update constraints, test cases, tests, algorithm discussion, and code. 
graphs_trees/bst_successor/bst_successor_challenge.ipynb