/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        int len = lists.size();
        ListNode* ret = new ListNode(-1);
        ListNode* ans = ret;
        priority_queue<pair<int, int>, vector<pair<int, int>>, greater<pair<int, int>>> orderedNodes;
        for (int i = 0; i < len; i++) {
            if (lists[i]) orderedNodes.push({lists[i]->val, i}); 
        }
        int idx = -1;
        while (!orderedNodes.empty()) {
            ret->next = new ListNode(orderedNodes.top().first);
            ret = ret->next;
            idx = orderedNodes.top().second;
            orderedNodes.pop();
            lists[idx] = lists[idx]->next;
            if (lists[idx]) orderedNodes.push({lists[idx]->val, idx}); 
        }
        return ans->next;
    }
};