Merge K Sorted Lists

思路：将每个链表的头存在最小堆中，每次取堆首，然后将取出节点的下一个放入堆中中。

class cmp{
public:
     bool operator()(ListNode* l1,ListNode* l2)
    {
        return l1->val>l2->val;
    }
 };
class Solution {
public:
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        ListNode* head,*ptr1,*ptr2;
        priority_queue<ListNode*,vector<ListNode*>,cmp> minHeap;
        for(int i=0;i<lists.size();i++)
            if(lists[i]!=NULL)
                minHeap.push(lists[i]);
        if(minHeap.empty())
            return NULL;
        head = ptr1 = ptr2= minHeap.top();
        minHeap.pop();
        if(ptr1->next!=NULL)
            minHeap.push(ptr1->next);
            
        while(!minHeap.empty())
        {
            ptr1 = minHeap.top();
           // cout<<ptr1->val<<endl;
            ptr2->next = ptr1;
            ptr2 = ptr2->next;
            minHeap.pop();
            if(ptr1->next==NULL)
                continue;
            minHeap.push(ptr1->next);
        }
        return head;
    }
};