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    "https://leetcode.com/problems/flatten-binary-tree-to-linked-list\n",
    "\n",
    "\n",
    "Runtime: 40 ms, faster than 68.21% of Python3 online submissions for Flatten Binary Tree to Linked List.\n",
    "Memory Usage: 15.2 MB, less than 75.33% of Python3 online submissions for Flatten Binary Tree to Linked List.\n",
    "\n",
    "\n",
    "```python\n",
    "class Solution:\n",
    "    def flatten(self, root: Optional[TreeNode]) -> None:\n",
    "        \"\"\"\n",
    "        Do not return anything, modify root in-place instead.\n",
    "        \"\"\"\n",
    "        #9:09\n",
    "        nodeList = []\n",
    "        def travel(node):\n",
    "            current = node\n",
    "            if current == None:\n",
    "                return\n",
    "            nodeList.append(node)\n",
    "            travel(node.left)\n",
    "            travel(node.right)\n",
    "        \n",
    "        travel(root)\n",
    "        \n",
    "        if len(nodeList) == 0: return None;\n",
    "        \n",
    "        head = nodeList[0]\n",
    "        for index, node in enumerate(nodeList):\n",
    "            if index+1 < len(nodeList):\n",
    "                node.left = None\n",
    "                node.right = nodeList[index+1]\n",
    "            if index == len(nodeList) - 1:\n",
    "                node.left = None\n",
    "                node.right = None\n",
    "        return head\n",
    "        #9:12\n",
    "        #debug until 9:15\n",
    "```"
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