LeetCode-Merge k Sorted Lists

哎，这两天都是在做水题，做水题都还一点状态都木有，
经常一道题调好久~~~
哎，好拙计！
 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         // Start typing your C/C++ solution below
13         // DO NOT write int main() function
14         if (lists.empty()) {
15             return NULL;
16         }
17         ListNode *res = NULL;
18         ListNode *pre;
19         ListNode *cur;
20         vector<ListNode *> prelist = lists;
21         bool first = false;
22         while (!prelist.empty()) {
23             bool val = false;
24             vector<ListNode *> curlist;
25             for (size_t i = 0; i < prelist.size(); ++i) {
26                 if (prelist[i] == NULL) {
27                     continue;
28                 }
29                 if (!val) {
30                     val = true;
31                     cur = prelist[i];
32                     continue;
33                 }
34                 if (prelist[i]->val < cur->val) {
35                     curlist.push_back(cur);
36                     cur = prelist[i];
37                 }
38                 else {
39                     curlist.push_back(prelist[i]);
40                 }
41             }
42             if(!val) {
43                 if (res == NULL) {
44                     return NULL;
45                 }
46                 break;
47             }
48             curlist.push_back(cur->next);
49             prelist = curlist;
50             if (!first) {
51                 first = true;
52                 res = cur;
53                 pre = cur;
54             }
55             else {
56                 pre->next = cur;
57                 pre = cur;
58             }
59         }
60         if (res == NULL) {
61             return res;
62         }
63         cur->next = NULL;
64         return res;
65     }
66 };