2020-07-23_binary-search-tree-second-largest-value.ipynb

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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "This problem was asked by Dropbox.\n",
    "\n",
    "Given the root to a binary search tree, find the second largest node in the tree."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "metadata": {},
   "outputs": [],
   "source": [
    "class BST:\n",
    "    def __init__(self, val, left=None, right=None):\n",
    "        self.val = val\n",
    "        self.left = left\n",
    "        self.right = right\n",
    "    \n",
    "    def second_largest_node(self):\n",
    "        if self.right is None:\n",
    "            return self.left.largest_node()\n",
    "        elif self.right.right is None:\n",
    "            return self\n",
    "        else:\n",
    "            return self.right.second_largest_node()\n",
    "    \n",
    "    def largest_node(self):\n",
    "        if self.right is None:\n",
    "            return self\n",
    "        else:\n",
    "            return self.right.largest_node()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "13"
      ]
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     "execution_count": 7,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "t = BST(5, BST(2, BST(1), BST(3)), BST(10, BST(9), BST(13, None, BST(15))))\n",
    "t.second_largest_node().val"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 13,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "13"
      ]
     },
     "execution_count": 13,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "t = BST(5, BST(2, BST(1), BST(3)), BST(10, BST(9), BST(13, BST(11, None, BST(12)), BST(15))))\n",
    "t.second_largest_node().val"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "2"
      ]
     },
     "execution_count": 8,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "t = BST(5, BST(2))\n",
    "t.second_largest_node().val"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 11,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "4"
      ]
     },
     "execution_count": 11,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "t = BST(5, BST(2, None, BST(4)))\n",
    "t.second_largest_node().val"
   ]
  }
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