Approach: Collect values, compare

def isPalindrome(self, head):
    vals = []
    while head:
        vals.append(head.val)
        head = head.next
    return vals == vals[::-1]

Complexity: O(n) time, O(n) space.

Full Solution — all languages

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next
class Solution:
    def isPalindrome(self, head):
        vals = []
        while head:
            vals.append(head.val)
            head = head.next
        return vals == vals[::-1]

import java.util.*;
class ListNode {
    int val;
    ListNode next;
    ListNode(int x) {
        val = x;
    }
}
class Solution {
    public boolean isPalindrome(ListNode head) {
        java.util.List<Integer> vals = new java.util.ArrayList<>();
        while (head != null) {
            vals.add(head.val);
            head = head.next;
        }
        int i = 0, j = vals.size() - 1;
        while (i < j) {
            if (!vals.get(i).equals(vals.get(j))) return false;
            i++;
            j--;
        }
        return true;
    }
}

#include <bits/stdc++.h> using namespace std;
struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x): val(x), next(nullptr) {
    }
};
class Solution {
    public: bool isPalindrome(ListNode* head) {
        vector<int> vals;
        while (head) {
            vals.push_back(head->val);
            head = head->next;
        }
        int i = 0, j = (int)vals.size() - 1;
        while (i < j) {
            if (vals[i] != vals[j]) return false;
            i++;
            j--;
        }
        return true;
    }
};

/** Node shape: { val, next }. Build/return plain objects or use this ctor. */ function ListNode(val, next) {
    this.val = (val === undefined ? 0 : val);
    this.next = (next === undefined ? null : next);
}
/**
 * @param {ListNode} head
 * @return {boolean}
 */ var isPalindrome = function(head) {
    const vals = [];
    while (head) {
        vals.push(head.val);
        head = head.next;
    }
    for (let i = 0, j = vals.length - 1; i < j; i++, j--) if (vals[i] !== vals[j]) return false;
    return true;
};

Palindrome Linked List

Example 1

Example 2

Constraints