Initialize of customize priority queue:

  1. define a struct of compare function.
  2. note the init method of pq.
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struct compare{
    bool operator()(ListNode *a, ListNode *b){
        return a->val> b->val;
    }
};

priority_queue<ListNode*, vector<ListNode*>, compare> pq;

reference:
http://www.cplusplus.com/reference/queue/priority_queue/priority_queue/