No.148 Sort List

 1 struct ListNode
 2 {
 3     int val;
 4     ListNode *next;
 5     ListNode(int x):val(x),next(NULL){}
 6 };
 7 class Solution
 8 {
 9 public:
10     ListNode* sortList(ListNode* head)
11     {//单链表排序：要求时间复杂度为O(nlogn),空间复杂度为O(1)
12         if(head == NULL || head->next == NULL)//终止条件
13             return head;
14 
15         //快慢指针找到中间节点
16         ListNode *fast = head;
17         ListNode *slow = head;
18         //将链表分为两段，因为fast快，所以以它为判断条件
19         while(fast->next != NULL && fast->next->next != NULL)
20         {
21             slow = slow->next;
22             fast = fast->next->next;
23         }
24 
25         ListNode *second = slow->next;
26         slow->next = NULL;//断开链表
27         //错：sortList(head);
28         //错：sortList(second);
29         //错：MergeSortedList(head, second);
30         //错：return head;
31         ListNode *l1= sortList(head);
32         ListNode *l2 = sortList(second);
33         return MergeSortedList(l1, l2);
34     }
35 
36     ListNode* MergeSortedList(ListNode *first, ListNode *second)
37     {
38         if(first == NULL)
39             return second;
40         if(second == NULL)
41             return first;
42 
43         ListNode *head, *current;
44         if(first->val <= second->val)
45         {
46             head = first;
47             first = first->next;
48         }
49         else
50         {
51             head = second;
52             second = second->next;
53         }
54         current = head;
55 
56         while(first!= NULL && second != NULL)
57         {
58             if(first->val <= second->val)
59             {
60                 current->next = first;
61                 first = first->next;
62             }
63             else
64             {
65                 current->next = second;
66                 second = second->next;
67             }
68             current = current->next;
69         }
70 /*
　　　　　　//注意与合并有序数组的区别
71         while(first != NULL)
72         {
73             current->next = first;
74             first = first->next;
75             current = current->next;
76         }
77         while(second != NULL)
78         {
79             current->next = second;
80             second = second->next;
81             current = current->next;
82         }
83 */
84         if(first)
85             current->next = first;
86         if(second)
87             current->next = second;
88 
89         return head;
90     }
91 };