LeetCode之“链表”：Reverse Nodes in k-Group

 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* reverseList(ListNode* head, int k)
12     {
13         if(!head || !head->next)
14             return head;
15             
16         int len = 0;
17         ListNode *start = head;
18         while(start)
19         {
20             len++;
21             start = start->next;
22             if(len == k)
23                 break;
24         }
25         if(len < k)
26             return head;
27         
28         ListNode *postList = start;
29         start = head;
30         ListNode *prev = nullptr;
31         while(start != postList)
32         {
33             ListNode *next = start->next;
34             start->next = prev;
35             prev = start;
36             start = next;
37         }
38         
39         start = prev;
40         while(start->next)
41             start = start->next;
42         start->next = postList;
43         
44         return prev;
45     }
46 
47     ListNode* reverseKGroup(ListNode* head, int k) {
48         ListNode *dummy = new(nothrow) ListNode(INT_MIN);
49         assert(dummy);
50         dummy->next = head;
51         ListNode *preNode = dummy;
52         while(true)
53         {
54             preNode->next = reverseList(head, k);
55             head = preNode->next;
56             int cnt = 0;
57             while(head)
58             {
59                 cnt++;
60                 head = head->next;
61                 preNode = preNode->next;
62                 if(cnt == k)
63                     break;
64             }
65             
66             if(cnt < k)
67             {
68                 head = dummy->next;
69                 delete dummy;
70                 dummy = nullptr;
71                 return head;
72             }
73         }
74     }
75 };