234. Palindrome Linked List

Given a singly linked list, determine if it is a palindrome.

Example 1:

Input: 1->2
Output: false

Example 2:

Input: 1->2->2->1
Output: true

Follow up:
Could you do it in O(n) time and O(1) space?

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool isPalindrome(ListNode* head) {
        if ( head == NULL  )
            return true;
        if ( head->next == NULL )
            return true;
        ListNode * slow = head;
        ListNode * fast = head;
        while(fast->next!=NULL&&fast->next->next!=NULL)
        {
            slow = slow->next;
            fast = fast->next->next;
        }
        slow->next = reverselist(slow->next);
        slow = slow->next;
        while(slow&&head)
        {
            if( slow->val != head->val )
                return false;
            slow = slow->next;
            head = head->next;
        }
        return true;
    }
    
    ListNode * reverselist( ListNode * head )
    {
        if ( head == NULL || head->next == NULL )
            return head;
        ListNode *pre = NULL;
        ListNode *cur = head;
        while(cur)
        {
            ListNode *next = cur->next;
            cur->next = pre;
            pre = cur;
            cur = next;
        }
        return pre;
    }
    
};