234. Palindrome Linked List

Easy

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?

package leetcode.easy;

/**
* Definition for singly-linked list. public class ListNode { int val; ListNode
* next; ListNode(int x) { val = x; } }
*/
public class PalindromeLinkedList {
public boolean isPalindrome(ListNode head) {
if (head == null) {
return true;
}
java.util.List<Integer> list = new java.util.ArrayList<>();
while (head != null) {
list.add(head.val);
head = head.next;
}
int end = list.size() - 1;
for (int i = 0; i < end; i++, end--) {
if (!list.get(i).equals(list.get(end))) {
return false;
}
}
return true;
} @org.junit.Test
public void test1() {
ListNode ln1 = new ListNode(1);
ListNode ln2 = new ListNode(2);
ln1.next = ln2;
ln2.next = null;
System.out.println(isPalindrome(ln1));
} @org.junit.Test
public void test2() {
ListNode ln1 = new ListNode(1);
ListNode ln2 = new ListNode(2);
ListNode ln3 = new ListNode(2);
ListNode ln4 = new ListNode(1);
ln1.next = ln2;
ln2.next = ln3;
ln3.next = ln4;
ln4.next = null;
System.out.println(isPalindrome(ln1));
}
}

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