Sort a linked list in O(n log n) time using constant space complexity.

題目要求給定時間復雜度對鏈表進行排序，但是我個人認為題目要求不是很合理，使用歸並排序時間復雜度滿足要求，但是空間復雜度並不為常數，所以個人認為這道題不嚴謹，說實話，對鏈表進行歸並排布的代碼涉及到遞歸，我還不是很熟悉，先貼上吧，日後再認真學習：

 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 || !head->next)
13             return head;
14         ListNode *fast = head, *slow = head, *pre = head;
15         while (fast && fast->next)
 
16         {
17             pre = slow;
18             slow = slow->next;
19             fast = fast->next->next;
20         }
21         pre->next = nullptr;
22         return merge(sortList(head), sortList(slow));
23     }
24     
25     ListNode *merge(ListNode* l1, ListNode *l2)
26     {
 
27         ListNode *dummy = new ListNode(-1);
28         ListNode *cur = dummy;
29         while (l1 && l2)
30         {
31             if (l1->val < l2->val)
32             {
33                 cur->next = l1;
34                 l1 = l1->next;
35             }
36             else
37             {
38                 cur->next = l2;
39                 l2 = l2->next;
40             }
41             cur = cur->next;
42         }
43         if (l1)
44             cur->next = l1;
45         if (l2)
46             cur->next = l2;
47         return dummy->next;
48     }
49 };

LeetCode 148. Sort List