MergeKSortedLists_Pqueue.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) {
        int len_l = lists.size();
        ListNode head(0);
        ListNode *h = &head;
        auto comp = [](ListNode* l1, ListNode* l2){
            return l1->val> l2->val;
        };
        priority_queue<ListNode*, std::vector<ListNode*>, decltype(comp)> q(comp);
        
        for(int i = 0; i < len_l; i++){
            if(lists[i])
                q.push(lists[i]);
        }
        while(q.empty() == false){
            h->next = q.top();
            q.pop();
            h = h->next;
            if(h->next != NULL)
                q.push(h->next); 
        }
        return head.next;
    }
};