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* sortList(ListNode* head) {
12         if(head == NULL)
13             return NULL;
14         ListNode *p = head;
15         int len = 0;
16         while(p) {
17             len++;
18             p = p->next;
19         }
20
21         ListNode *temp = new ListNode(0);
22         temp->next = head;
23
24         int interval;
25         for(interval = 1; interval <= len; interval *= 2) { //从下往上归并
26
27             ListNode *pre = temp;
28             ListNode *first = temp->next;
29             ListNode *second = temp->next;
30
31             while(first && second) {
32                 int i = 0;
33                 while(i < interval && second) {
34                     second = second->next;
35                     i++;
36                 }
37                 int fvisit = 0;
38                 int svisit = 0;
39                 while(fvisit < interval && svisit < interval && first && second) {
40                     if(first->val < second->val) { //前指针较小，则把前指针装入放在前头
41                         pre->next = first;
42                         pre = first;
43                         first = first->next;
44                         fvisit++;
45                     }
46                     else {
47                         pre->next = second;
48                         pre = second;
49                         second = second->next;
50                         svisit++;
51                     }
52                 }
53                 while(fvisit < interval && first) {//剩下的较大的就依次接上就好
54                     pre->next = first;
55                     pre = first;
56                     first = first->next;
57                     fvisit++;
58                 }
59                 while(svisit < interval && second) {
60                     pre->next = second;
61                     pre = second;
62                     second = second->next;
63                     svisit++;
64                 }
65                 pre->next = second; //往后继续直到第一次排序结束
66                 first = second;
67             }
68         }
69         ListNode *ans = temp->next;
70         delete temp;
71         return ans;
72     }
73 };