Flatten a Multilevel Doubly Linked List LeetCode solution in C++

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class Solution {
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 public:
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  Node *flatten(Node *head) {
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    if (head == NULL) return head;
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    Node *temp = head;
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    stack<Node *> stk;
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    while (temp->next != NULL || temp->child != NULL || stk.size() != 0) {
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      if (temp->next == NULL && temp->child == NULL && stk.size()) {
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        Node *a = stk.top();
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        stk.pop();
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        temp->next = a;
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        a->prev = temp;
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      }
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      if (temp->child != NULL) {
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        if (temp->next != NULL) {
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          Node *a = temp->next;
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          a->prev = NULL;
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          stk.push(a);
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        }
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        temp->next = temp->child;
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        temp->next->prev = temp;
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        temp->child = NULL;
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      }
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      temp = temp->next;
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    }
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    return head;
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  }
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};
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