Merge K Sorted Lists

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     ListNode* mergeKLists(vector<ListNode*>& lists) {
12         if(lists.size() == 0) return NULL;
13         if(lists.size() == 1) return lists[0];
14         
15         return divide(lists, 0, lists.size() - 1);
16     }
17     ListNode* divide(vector<ListNode*> &lists, int left, int right){
18         if(left < right){
19             int m = (left + right) / 2;
20             return merge2lists(divide(lists, left, m), divide(lists, m + 1, right));
21         }
22         return lists[0];
23     }
24     ListNode *merge2lists(ListNode *l1, ListNode *l2) {
25         ListNode pre(-1);
26         
27         for(ListNode* p = &pre; l1 || l2; p = p->next){
28             int val1 = l1 == nullptr ? INT_MAX : l1->val;
29             int val2 = l2 == nullptr ? INT_MAX : l2->val;
30             
31             if(val1 > val2){
32                 p->next = l2;
33                 l2 = l2->next;
34             }
35             else{
36                 p->next = l1;
37                 l1 = l1->next;
38             }
39         }
40         return pre.next;
41     }
42 };