Merge K Sorted Lists

/**
 * 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) {
        ListNode *head = nullptr, *curr = nullptr;

        using Node = pair<int, ListNode*>;
        priority_queue<Node, vector<Node>, greater<Node>> heap;

        for (ListNode *node: lists) {
            if (node != nullptr) {
                heap.emplace(node->val, node);
            }
        }

        while (!heap.empty()) {
            ListNode *top = heap.top().second;
            heap.pop();

            if (head == nullptr) {
                head = curr = top;
            } else {
                curr->next = top;
                curr = top;
            }

            if (top->next != nullptr) {
                heap.emplace(top->next->val, top->next);
            }

            top->next = nullptr;
        }

        return head;
    }
};