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    "<h1>226. Invert Binary Tree</h1>\n",
    "<hr>\n",
    "\n",
    "<!--Copy Paste Leetcode statement between-->\n",
    "<p>Invert a binary tree.</p>\n",
    "\n",
    "<p><strong>Example:</strong></p>\n",
    "\n",
    "<p>Input:</p>\n",
    "\n",
    "<pre>     4\n",
    "   /   \\\n",
    "  2     7\n",
    " / \\   / \\\n",
    "1   3 6   9</pre>\n",
    "\n",
    "<p>Output:</p>\n",
    "\n",
    "<pre>     4\n",
    "   /   \\\n",
    "  7     2\n",
    " / \\   / \\\n",
    "9   6 3   1</pre>\n",
    "\n",
    "<p>&nbsp;</p>\n",
    "<a href=\"https://leetcode.com/problems/invert-binary-tree/\">Source</a> \n",
    "<hr>"
   ]
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   "cell_type": "code",
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   "source": [
    "# Definition for a binary tree node.\n",
    "class TreeNode:\n",
    "    def __init__(self, val=0, left=None, right=None):\n",
    "        self.val = val\n",
    "        self.left = left\n",
    "        self.right = right"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "<h4>Code</h4>"
   ]
  },
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   "cell_type": "code",
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   "outputs": [],
   "source": [
    "def invert_tree(root):\n",
    "    \"\"\"Recursive approach\"\"\"\n",
    "    if root and (root.left or root.right):\n",
    "        root.left, root.right = root.right, root.left\n",
    "        invert_tree(root.left)\n",
    "        invert_tree(root.right)\n",
    "    return root"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "<hr>\n",
    "<h4>Follow up:</h4>\n",
    "<p>Solve it both recursively and iteratively.</p>\n",
    "\n",
    "<h4>Code</h4>"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {},
   "outputs": [],
   "source": [
    "def invert_tree(root):\n",
    "    \"\"\"Iterative approach\"\"\"\n",
    "    if not root:\n",
    "        return None\n",
    "\n",
    "    stack = [root]\n",
    "    while stack:\n",
    "        node = stack.pop()\n",
    "        if node:\n",
    "            node.left, node.right = node.right, node.left\n",
    "            stack.append(node.left)\n",
    "            stack.append(node.right)\n",
    "    return root"
   ]
  }
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