Merge K Sorted Lists

 1 class cmp{
 2 public:
 3      bool operator()(ListNode* l1,ListNode* l2)
 4     {
 5         return l1->val>l2->val;
 6     }
 7  };
 8 class Solution {
 9 public:
10     ListNode* mergeKLists(vector<ListNode*>& lists) {
11         ListNode* head,*ptr1,*ptr2;
12         priority_queue<ListNode*,vector<ListNode*>,cmp> minHeap;
13         for(int i=0;i<lists.size();i++)
14             if(lists[i]!=NULL)
15                 minHeap.push(lists[i]);
16         if(minHeap.empty())
17             return NULL;
18         head = ptr1 = ptr2= minHeap.top();
19         minHeap.pop();
20         if(ptr1->next!=NULL)
21             minHeap.push(ptr1->next);
22             
23         while(!minHeap.empty())
24         {
25             ptr1 = minHeap.top();
26            // cout<<ptr1->val<<endl;
27             ptr2->next = ptr1;
28             ptr2 = ptr2->next;
29             minHeap.pop();
30             if(ptr1->next==NULL)
31                 continue;
32             minHeap.push(ptr1->next);
33         }
34         return head;
35     }
36 };