Merge k Sorted Lists LeetCode solution in C++

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class Solution {
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 public:
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  struct compare {
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    bool operator()(ListNode *&a, ListNode *&b) {
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      return a->val > b->val;
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    }
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  };
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  ListNode *mergeKLists(vector<ListNode *> &lists) {
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    priority_queue<ListNode *, vector<ListNode *>, compare> minh;
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    for (int i = 0; i < lists.size(); i++) {
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      if (lists[i] != NULL) minh.push(lists[i]);
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    }
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    ListNode *head = new ListNode(0);
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    ListNode *temp = head;
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    while (minh.size() > 0) {
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      ListNode *p = minh.top();
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      minh.pop();
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      temp->next = new ListNode(p->val);
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      temp = temp->next;
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      if (p->next != NULL) minh.push(p->next);
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    }
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    return head->next;
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  }
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};