234. Palindrome Linked List

https://leetcode.com/problems/palindrome-linked-list/

Problem

Given a singly linked list, determine if it is a palindrome.

Example 1:

Input: 1->2
Output: false

Example 2:

Input: 1->2->2->1
Output: true

Follow up: Could you do it in O(n) time and O(1) space?

Solution

/**
 * 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, *fast = head;
        while (fast != nullptr) {
            fast = fast->next;
            if (fast != nullptr) fast = fast->next;
            slow = slow->next;
        }
        ListNode *tail = reverse(slow);
        int i = 0;
        while (tail != nullptr) {
            if (head->val != tail->val) return false;
            head = head->next;
            tail = tail->next;
        }
        return true;
    }
    ListNode* reverse(ListNode* head) {
        if (head == nullptr) return nullptr;
        ListNode* new_next = head->next;
        head->next = nullptr;
        return reverse(head, new_next);
    } 
    ListNode* reverse(ListNode *current, ListNode *next) {
        if (next == nullptr) return current;
        ListNode* new_next = next->next;
        next->next = current;
        return reverse(next, new_next);
    }
};

• #linkedlist