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) {
    
    if (head == NULL || 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;
    }

    ListNode* prev = NULL;
    ListNode* curr = slow->next;
    ListNode* next = NULL;

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

    ListNode* secondHalf = prev;

    ListNode* firstHalf = head;
    while (secondHalf != NULL) {
        if (firstHalf->val != secondHalf->val) {
            return false;
        }
        firstHalf = firstHalf->next;
        secondHalf = secondHalf->next;
    }

    return true;
    }
};