0082_P234_Palindrome_Linked_List.cpp

/* 
Day: 82
Problem Number: 234 (https://leetcode.com/problems/palindrome-linked-list)
Date: 22-03-2024
Description:
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

Constraints:
* The number of nodes in the list is in the range [1, 10^5].
* 0 <= Node.val <= 9
 
Follow up: Could you do it in O(n) time and O(1) space?

Code:  */
/**
 * 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;
        ListNode* fast = head->next;
        while (fast && fast->next) {
            slow = slow->next;
            fast = fast->next->next;
        }
        ListNode* pre = nullptr;
        ListNode* cur = slow->next;
        while (cur) {
            ListNode* t = cur->next;
            cur->next = pre;
            pre = cur;
            cur = t;
        }
        while (pre) {
            if (pre->val != head->val) return false;
            pre = pre->next;
            head = head->next;
        }
        return true;
    }
};