0023.cpp

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */

class Solution {
public:
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        ListNode* dummy = new ListNode(0);
        ListNode* curr = dummy;
        priority_queue<ListNode*, vector<ListNode*>, compareListNode> pq;

        for (auto list : lists)
            if (list) pq.push(list);

        while (!pq.empty()) {
            curr->next = pq.top();
            pq.pop();
            curr = curr->next;
            if (curr->next) pq.push(curr->next);
        }

        return dummy->next;
    }

private:
    struct compareListNode {
        bool operator()(const ListNode* l1, const ListNode* l2) {
            return l1->val > l2->val;
        }
    };
};