Merge k Sorted Lists

 1 #include <iostream>
 2 #include <algorithm>  //std::make_heap(),std::pop_heap(),std::push_heap,std::sort_heap()
 3 #include <vector>
 4 
 5 using std::vector;
 6 
 7 struct ListNode {
 8     int val;
 9     ListNode *next;
10     ListNode(int x) : val(x), next(NULL) {}
11 };
12 
13 class Solution {
14 public:
15     ListNode* mergeKLists(vector<ListNode*>& lists) {
16         if(lists.empty())  return nullptr;
17         int n = lists.size();
18         vector<ListNode*> minHeap;
19         minHeap.reserve(n);
20         for(int i=0;i<n;i++)
21             minHeap.push_back(lists[i]);
22         make_heap(minHeap.begin(),minHeap.end(),greater);  //construct the first min heap
23 
24         ListNode head = ListNode(-1);
25         ListNode *p= &head;
26         while(minHeap.front() != nullptr)
27         {
28             p->next = minHeap.front(); 
29             pop_heap(minHeap.begin(), minHeap.end(),greater);
30             minHeap.pop_back();
31             minHeap.push_back(p->next->next);
32             push_heap(minHeap.begin(),minHeap.end(),greater);
33             p=p->next;
34         }  
35         return head.next;  
36     }
37 private:
38     static bool greater(ListNode* l1,ListNode* l2){
39         if(l1 == nullptr)  return true;
40         else if(l2 == nullptr)  return false;
41         else return l1->val >= l2->val;
42     }  
43 };
44 
45 int main()
46 {
47     vector<ListNode*> lists;
48     ListNode* list1 = new ListNode(1);
49     list1->next = new ListNode(3);
50     list1->next->next = new ListNode(6);
51 
52     ListNode* list2 = new ListNode(2);
53     list2->next = new ListNode(4);
54     list2->next->next = new ListNode(5);
55 
56     ListNode* list3 = new ListNode(100);
57 
58     lists.push_back(list1);
59     lists.push_back(list2);
60     lists.push_back(list3);
61 
62     Solution s;
63     ListNode *head;
64     head = s.mergeKLists(lists);
65     while(head!= nullptr)
66     {
67         std::cout << head->val << "->";
68         head = head->next;
69     }
70     return 0;
71 }