[leetcode] 206.Palindrome Linked List

 1 bool isPalindrome(ListNode* head) 
 2     {
 3         if (!head || !head->next)
 4             return true;
 5         
 6         ListNode* fast = head;
 7         ListNode* slow = head;
 8         while (fast && fast->next)
 9         {
10             fast = fast->next->next;
11             slow = slow->next;
12         }
13         
14         if (fast) // odd nodes
15         {
16             slow->next = reverseList(slow->next);
17             slow = slow->next;
18         }
19         else
20         {
21             slow = reverseList(slow);
22         }
23         
24         while (slow)
25         {
26             if (head->val != slow->val)
27                 return false;
28             head = head->next;
29             slow = slow->next;
30         }
31         
32         return true;
33     }
34     
35     ListNode* reverseList(ListNode* head) 
36     {
37         if (!head || !head->next)
38             return head;
39         
40         ListNode* p = head, *q = p->next, *r = q->next;
41         while (r != NULL)
42         {
43             q->next = p;
44             p = q;
45             q = r;
46             r = r->next;
47         }
48         q->next = p;
49         head->next = NULL;
50         head = q;
51         
52         return head;
53     }