palindromeLL.cpp

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 {
    private:
    ListNode* getMid(ListNode* head){
        ListNode* slow = head;
        ListNode* fast = head->next;

        while(fast != NULL && fast->next != NULL){
            fast = fast->next->next;
            slow = slow->next;
        }
        return slow;
    }

    ListNode* reverse(ListNode* head){
        ListNode* curr = head;
        ListNode* prev = NULL;
        ListNode* next = NULL;

        while(curr != NULL){
            next = curr->next;
            curr->next = prev;
            prev = curr;
            curr = next;
        }
        return prev;
    }

public:
    bool isPalindrome(ListNode* head) {
        if(head == NULL || head->next == NULL){
            return true;
        }
        
        ListNode* mid = getMid(head);
        
        //reversing second half of LL
        ListNode* temp = mid->next;
        mid->next = reverse(temp);

        //comapring both parts of LL
        ListNode* head1 = head;
        ListNode* head2 = mid->next;

        while(head2 != NULL){
            if(head1->val != head2->val){
                return false;
            }
            head1 = head1->next;
            head2 = head2->next;
        }

        //reverse again to get back original LL
        //this is optional
        temp = mid->next;
        mid->next = reverse(temp);

        return true;
    }
};