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

鏈表，快慢指針找中點，歸並排序。

註意判斷條件fast->next!=NULL&&fast->next->next!=NULL，若為fast!=NULL&&fast->next!=NULL則會出現內存溢出

 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 || head->next==NULL)
13             return head;
14         ListNode* slow, *fast;
15         slow=head;
16         fast=head;
17         while
 
(fast->next!=NULL&&fast->next->next!=NULL){
18             slow=slow->next;
19             fast=fast->next->next;
20         }
21         fast=slow;
22         slow=slow->next;
23         fast->next=NULL;
24         fast=sortList(head);
25         slow=sortList(slow);
 
26         return merge(fast,slow);
27         
28     }
29     ListNode *merge(ListNode *left, ListNode *right){
30         ListNode *res,*temp;
31         if(left==NULL){
32             res=right;
33             return res;
34         }
35         if(right==NULL){
36             res=left;
37             return res;
38         }
39         if(left->val<=right->val){
40             res=left;
41             left=left->next;
42         }else{
43             res=right;
44             right=right->next;
45         }
46         temp=res;
47         while(left!=NULL&&right!=NULL){
48             if(left->val<=right->val){
49                 temp->next=left;
50                 left=left->next;
51             }else{
52                 temp->next=right;
53                 right=right->next;
54             }
55             temp=temp->next;
56             
57         }
58         if(left!=NULL){
59             temp->next=left;
60         }
61         if(right!=NULL){
62             temp->next=right;
63         }
64         return res;
65     }
66 };

sort-list——鏈表、快慢指針找中間、歸並排序