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   "source": [
    "https://leetcode.com/problems/binary-tree-preorder-traversal/\n",
    "\n",
    "\n",
    "Runtime: 20 ms, faster than 99.20% of Python3 online submissions for Binary Tree Preorder Traversal.\n",
    "Memory Usage: 14.3 MB, less than 16.12% of Python3 online submissions for Binary Tree Preorder Traversal.\n",
    "\n",
    "\n",
    "```python\n",
    "class Solution:\n",
    "    def preorderTraversal(self, root: TreeNode) -> List[int]:\n",
    "        self.l = []\n",
    "        self.travel(root)\n",
    "        return self.l\n",
    "    \n",
    "    def travel(self, node):\n",
    "        current = node\n",
    "        if current == None:\n",
    "            return\n",
    "        self.l.append(current.val)\n",
    "        self.travel(node.left)\n",
    "        self.travel(node.right)\n",
    "```"
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