234.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 {
public:
    bool isPalindrome(ListNode* head) {
        ListNode* slow = head, *fast = head;

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

        ListNode* prev = nullptr;
        while (slow) {
            ListNode* temp = slow->next;
            slow->next = prev;
            prev = slow;
            slow = temp;
        }

        ListNode* left = head, *right = prev;
        while (right) {
            if (left->val != right->val) return false;
            left = left->next;
            right = right->next;
        }

        return true;
    }
};
    
/* bool stackSol(ListNode* head) {
    stack<int> st;
    ListNode* dummy = head;

    while (dummy) {
        st.push(dummy->val);
        dummy = dummy->next;
    }

    dummy = head;
    while (dummy && dummy->val == st.top()) {
        st.pop();
        dummy = dummy->next;
    }

    return st.empty();
} */