PalindromeLinkedList.cpp

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    bool isPalindrome(ListNode* head) {
        ListNode* slow=head, *fast =head;

        while(fast && fast->next){
            slow = slow->next; 
            fast = fast->next->next;
        }
        if(fast != NULL && fast->next == NULL){
            slow = slow->next;
        }
        ListNode* prev = nullptr, *nex;
        while(slow != NULL && slow->next != NULL){
            nex = slow->next;
            slow->next = prev;
            prev = slow;
            slow = nex;
        }

        if(slow != NULL){
            slow->next = prev;
        }

        fast = head;
        while(slow && fast){
            if(slow->val != fast->val){
                return false;
            }
            slow = slow->next;
            fast = fast->next;
        }
        return true;
    }
};