Q11_palindrome_linked_list.cpp

// Given the head of a singly linked list, return true if it is a palindrome or false otherwise.

// Example 1:
// Input: head = [1,2,2,1]
// Output: true

// Example 2:
// Input: head = [1,2]
// Output: false

#include<bits/stdc++.h>
using namespace std;

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

        while(curr) {
            ListNode* next = curr->next;
            curr->next = prev;
            prev = curr;
            curr = next;
        }
        return prev;
    }
public:
    bool isPalindrome(ListNode* head) {
        if(!head || !head->next) return true;

        ListNode* slow = head;
        ListNode* fast = head;

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

        ListNode* runner1 = head;
        ListNode* runner2 = reverse(slow->next);

        while(runner1 && runner2) {
            if(runner1->val != runner2->val) return false;
            runner1 = runner1->next;
            runner2 = runner2->next;
        }
        return true;
    }
};

// Time Complexity  : O(n)
// Space Complexity : O(1)