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;
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));
}
}