      "   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>
    "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_solution.ipynb