reverse the 2nd half and check pallin linked list

/**
 * 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 true;
        ListNode *slow = head,*fast = head->next;
        while(fast&&fast->next){
            slow = slow->next;
            fast = fast->next->next;
        }
        ListNode *prev = slow;
        ListNode *pp = prev->next;
        prev->next = NULL;
        while(pp){
            ListNode *tmp = pp->next;
            pp->next = prev;
            prev = pp;
            pp = tmp;
        }
        while(head&&prev){
            if(head->val!=prev->val)return false;
            head = head->next;
            prev = prev->next;
        }
        return true;

    }
};