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    "https://leetcode.com/problems/flatten-a-multilevel-doubly-linked-list/\n",
    "\n",
    "\n",
    "Runtime: 8 ms, faster than 32.69% of C++ online submissions for Flatten a Multilevel Doubly Linked List.\n",
    "Memory Usage: 8.9 MB, less than 13.83% of C++ online submissions for Flatten a Multilevel Doubly Linked List.\n",
    "\n",
    "\n",
    "```cpp\n",
    "#include <vector>\n",
    "#include <algorithm>\n",
    "#include <string>\n",
    "#include <deque>\n",
    "#include <iostream>\n",
    "\n",
    "using namespace std;\n",
    "\n",
    "class Node {\n",
    "public:\n",
    "    int val;\n",
    "    Node* prev;\n",
    "    Node* next;\n",
    "    Node* child;\n",
    "};\n",
    "\n",
    "class Solution {\n",
    "public:\n",
    "    Node* flatten(Node* head) {\n",
    "        //10:01\n",
    "        vector<Node*> linkList;\n",
    "\n",
    "        deque<Node*> q;\n",
    "        q.push_front(head);\n",
    "        Node* node;\n",
    "        while (q.size()) {\n",
    "            node = q.front();\n",
    "            q.pop_front();\n",
    "            if (node != nullptr) {\n",
    "                if (node->next != nullptr) {\n",
    "                    q.push_front(node->next);\n",
    "                }\n",
    "                if (node->child != nullptr) {\n",
    "                    q.push_front(node->child);\n",
    "                }\n",
    "                linkList.push_back(node);\n",
    "            }\n",
    "        }\n",
    "\n",
    "        for (int i=0; i<linkList.size(); i++) {\n",
    "            node = linkList[i];\n",
    "            if (i == 0) {\n",
    "                node->prev = nullptr;\n",
    "                node->next = linkList[i+1];\n",
    "            } else if (i == linkList.size()-1) {\n",
    "                node->prev = linkList[i-1];\n",
    "                node->next = nullptr;\n",
    "            } else {\n",
    "                node->prev = linkList[i-1];\n",
    "                node->next = linkList[i+1];\n",
    "            }\n",
    "            node->child = nullptr;\n",
    "            //cout << node->val << endl;\n",
    "        }\n",
    "\n",
    "        return head;\n",
    "        //10:31 done; 11:15 debug\n",
    "    }\n",
    "};\n",
    "```"
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