234. Palindrome Linked List

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

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) return 1;        ListNode* slow = head;        ListNode *fast = slow;        while(fast && fast->next && fast->next->next){            slow = slow->next;            fast = fast->next->next;        }                ListNode* head2 = slow->next;        slow->next = NULL;                ListNode* pre = NULL;        ListNode* cur = head2;        ListNode* nxt = NULL;        while(head2){            nxt = head2->next;            head2->next = pre;            pre = head2;            head2 = nxt;        }                head2 = pre;        while(head && head2){            if(head->val != head2->val) return 0;            head = head->next;            head2 = head2->next;        }                return 1;            }};