Sort List

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

nlogn的排序有快速排序、归并排序、堆排序。双向链表用快排比较适合，堆排序也可以用于链表，单向链表适合用归并排序。以下是用归并排序的代码：

 

 1 /**

 2  * Definition for singly-linked list.

 3  * class ListNode {

 4  *     int val;

 5  *     ListNode next;

 6  *     ListNode(int x) {

 7  *         val = x;

 8  *         next = null;

 9  *     }

10  * }

11  */

12  

13  

14  /**

15  * Definition for singly-linked list.

16  * struct ListNode {

17  * int val;

18  * ListNode *next;

19  * ListNode(int x) : val(x), next(NULL) {}

20  * };

21  */

22 

23 public class Solution {

24     public ListNode sortList(ListNode head) {

25         if(head == null || head.next == null) return head;

26         ListNode fast = head, slow = head;

27         while(fast.next != null && fast.next.next != null){

28             fast = fast.next.next;

29             slow = slow.next;

30         }

31         fast = slow.next;

32         slow.next = null;

33         fast = sortList(fast);// 后半段

34         slow = sortList(head);//前半段

35         return merge(slow, fast);

36     }

37     ListNode merge(ListNode head1, ListNode head2){

38         if(head1 == null)return head2;

39         if(head2 == null)return head1;

40         ListNode ret = null, cur = null ;

41         if(head1.val < head2.val){

42             ret = head1; 

43             head1 = head1.next;

44         }else{

45             ret = head2;

46             head2 = head2.next;

47         }

48         cur = ret;

49         while(head1 != null && head2 != null){

50             if(head1.val < head2.val){

51                 cur.next = head1;

52                 head1 = head1.next;

53             }else{

54                 cur.next = head2;

55                 head2 = head2.next;

56             }

57             cur = cur.next;

58         }

59         if(head1 != null) cur.next = head1;

60         if(head2 != null) cur.next = head2;

61         return ret;

62     }

63 }

 

 第二遍：

这次做的时候 有一句话不一样。while(fast != null && fast.next != null) 在最开始分组时， 结果导致runtime error。

Last executed input: {2,1}