Sort List

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

思路：题目要求O(n log n)的时间复杂度以及常空间复杂度，因此，使用归并排序策略。

 1 class Solution {
 2 public:
 3     ListNode *sortList( ListNode *head ) {
 4         if( !head || !head->next ) { return head; }
 5         if( !head->next->next ) {
 6             if( head->val > head->next->val ) {
 7                 int temp = head->val;
 8                 head->val = head->next->val;
 9                 head->next->val = temp;
10             }
11             return head;
12         }
13         ListNode *slow = head, *fast = head->next;
14         while( fast->next ) {
15             slow = slow->next;
16             fast = fast->next;
17             if( fast->next ) { fast = fast->next; }
18         }
19         fast = slow->next; slow->next = 0;
20         slow = sortList( head );
21         fast = sortList( fast );
22         return merge( slow, fast );
23     }
24 private:
25     ListNode *merge( ListNode *slow, ListNode *fast ) {
26         if( !slow ) { return fast; }
27         if( !fast ) { return slow; }
28         ListNode *head = 0, *end = 0;
29         if( slow->val <= fast->val ) {
30             head = end = slow; slow = slow->next;
31         } else {
32             head = end = fast; fast = fast->next;
33         }
34         while( slow && fast ) {
35             if( slow->val <= fast->val ) {
36                 end->next = slow;
37                 end = end->next;
38                 slow = slow->next;
39             } else {
40                 end->next = fast;
41                 end = end->next;
42                 fast = fast->next;
43             }
44         }
45         if( slow ) { end->next = slow; }
46         else { end->next = fast; }
47         return head;
48     }
49 };

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