Merge k Sorted Lists.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) {
        auto comp = [](const ListNode* n1, const ListNode* n2)
        {
            return n1->val > n2->val;  
        };
        
        priority_queue<ListNode*, vector<ListNode*>, decltype(comp)> pq(comp);
        for(const auto node : lists)if(node)pq.push(node);
        ListNode* head = nullptr, *tail = nullptr;
        while(pq.size())
        {
            auto curr = pq.top();
            pq.pop();
            if(!head)
            {
                head = curr;
                tail = curr;
            }
            else
            {
                tail->next = curr;
                tail = tail->next;
            }
            if(curr->next)
                pq.push(curr->next);
        }
        return head;
    }
};