LeetCode题解之 Merge k Sorted Lists

 1 ListNode* mergeKLists(vector<ListNode*>& lists) {
 2         
 3         ListNode *res = NULL;
 4         for (vector<ListNode*>::iterator it = lists.begin(); it != lists.end(); it++) {
 5             res = mergeTwoLists(res, *it);
 6         }
 7         
 8         return res;
 9     }
10     
11     ListNode *mergeTwoLists(ListNode *list1, ListNode *list2)
12     {
13         if (list1 == NULL)
14             return list2;
15         if (list2 == NULL)
16             return list1;
17         
18         ListNode *p1 = list1;
19         ListNode *p2 = list2;
20         ListNode *dummy = new ListNode(0);
21         ListNode *p = dummy;
22         while (p1 != NULL && p2 != NULL) {
23             if (p1->val <= p2->val) {
24                 p->next = p1;
25                 p1 = p1->next;
26             } else {
27                 p->next = p2;
28                 p2 = p2->next;
29             }
30             p = p->next;
31         }
32         
33         while (p1 != NULL) {
34             p->next = p1;
35             p = p->next;
36             p1 = p1->next;
37         }
38         
39         while (p2 != NULL) {
40             p->next = p2;
41             p = p->next;
42             p2 = p2->next;
43         }
44         return dummy->next;
45     }