merge-k-sorted-lists

class Solution {
public:
    struct cmp{
        bool operator()(const ListNode*a,const ListNode*b){
            return a->val>b->val;
        }
    };
    
    ListNode *mergeKLists(vector<ListNode *> &lists) {
        priority_queue<ListNode*,vector<ListNode*>,cmp>q;
        for(auto l:lists)if(l)q.push(l);
        ListNode*dummy=new ListNode(-1);
        ListNode*head=dummy;
        while(!q.empty())
        {
            ListNode*top=q.top();
            q.pop();
            head->next=top;
            head=head->next;
            if(top->next)q.push(top->next);
        }
        return dummy->next;
    }
};