pre_order_sequentialize.ipynb

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   "source": [
    "# Easy Pre-Order Traversal Using \"`yield from`\""
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "![Pre-order traversal diagram](https://upload.wikimedia.org/wikipedia/commons/d/d4/Sorted_binary_tree_preorder.svg \"Pre-order traversal diagram\")"
   ]
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   "cell_type": "markdown",
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   "source": [
    "## F, B, A, D, C, E, G, I, H"
   ]
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   "source": [
    "Wikipedia gives this algorithm:\n",
    "> https://en.wikipedia.org/wiki/Tree_traversal#Pre-order\n",
    ">\n",
    "> 1. Display the data part of the root (or current node).\n",
    "> 2. Traverse the left subtree by recursively calling the pre-order function.\n",
    "> 3. Traverse the right subtree by recursively calling the pre-order function.\n",
    "\n",
    "But this is specific to binary trees.\n",
    "\n",
    "In general, this is the algorithm for a pre-order traversal:\n",
    "\n",
    "1. Display the data part of the current node.\n",
    "2. Traverse each subtree by recursively calling the pre-order function."
   ]
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  {
   "cell_type": "code",
   "execution_count": 1,
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   "source": [
    "# imports\n",
    "from typing import Iterator, Iterable"
   ]
  },
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    "# Let's use a dict to represent this tree\n",
    "#\n",
    "# Each node looks like this:\n",
    "# {\n",
    "#   'node': <value>,\n",
    "#   'branches': [list of nodes...],\n",
    "# }\n",
    "the_tree = {\n",
    "    'node': 'F',\n",
    "    'branches': [\n",
    "        {\n",
    "            'node': 'B',\n",
    "            'branches': [\n",
    "                {\n",
    "                    'node': 'A',\n",
    "                    'branches': [],\n",
    "                },\n",
    "                {\n",
    "                    'node': 'D',\n",
    "                    'branches': [\n",
    "                        {\n",
    "                            'node': 'C',\n",
    "                            'branches': [],\n",
    "                        },\n",
    "                        {\n",
    "                            'node': 'E',\n",
    "                            'branches': [],\n",
    "                        },\n",
    "                    ],\n",
    "                },\n",
    "            ],\n",
    "        },\n",
    "        {\n",
    "            'node': 'G',\n",
    "            'branches': [\n",
    "                {\n",
    "                    'node': 'I',\n",
    "                    'branches': [\n",
    "                        {\n",
    "                            'node': 'H',\n",
    "                            'branches': [],\n",
    "                        },\n",
    "                    ],\n",
    "                },\n",
    "            ],\n",
    "        },\n",
    "    ],\n",
    "}"
   ]
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   "source": [
    "def pre_order_sequentialize(tree: dict) -> Iterator[str]:\n",
    "    \"\"\"Yield the values of a tree in a pre-order traversal.\"\"\"\n",
    "    yield tree['node']  # display the data part of the current node\n",
    "    for branch in tree['branches']: # traverse each subtree\n",
    "        yield from pre_order_sequentialize(branch) # by recursively calling the pre-order function"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {
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   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "<generator object pre_order_sequentialize at 0x7f96bc1dae60>\n",
      "F\n",
      "B\n",
      "A\n"
     ]
    }
   ],
   "source": [
    "what_is_it = pre_order_sequentialize(the_tree)\n",
    "print(what_is_it)\n",
    "print(next(what_is_it))\n",
    "print(next(what_is_it))\n",
    "print(next(what_is_it))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {
    "collapsed": true
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   "outputs": [],
   "source": [
    "def pretty_print_sequence(seq: Iterable) -> str:\n",
    "    \"\"\"Return a nice string representation of a sequence.\n",
    "    \n",
    "    Turn [1, 2, 3] into \"1, 2, 3\".\n",
    "    \"\"\"\n",
    "    return ', '.join(seq)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "def pretty_print_tree(tree: dict) -> str:\n",
    "    \"\"\"Return a nice view of the pre-order sequentialization.\"\"\"\n",
    "    return pretty_print_sequence(pre_order_sequentialize(tree))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "F, B, A, D, C, E, G, I, H\n",
      "True\n"
     ]
    }
   ],
   "source": [
    "sequentialization = pretty_print_tree(the_tree)\n",
    "print(sequentialization)\n",
    "print(sequentialization == 'F, B, A, D, C, E, G, I, H')"
   ]
  }
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