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/**

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

*

* Follow up:

* Could you do it in O(n) time and O(1) space?

*/

/**

* Definition for singly-linked list.

* public class ListNode {

* int val;

* ListNode next;

* ListNode(int x) { val = x; }

* }

*/

public class PalindromeLinkedList234 {

public boolean isPalindrome(ListNode head) {

ListNode dummy = new ListNode(0);

ListNode f = head;

ListNode s = head;

while (s != null && f != null && f.next != null) {

ListNode t = s;

s = s.next;

f = f.next.next;

t.next = dummy.next;

dummy.next = t;

}

if (f != null) {

s = s.next;

}

dummy = dummy.next;

while (s != null && dummy != null) {

if (dummy.val != s.val) return false;

s = s.next;

dummy = dummy.next;

}

return true;

}

/**

* https://leetcode.com/problems/palindrome-linked-list/discuss/64501/Java-easy-to-understand

*/

public boolean isPalindrome2(ListNode head) {

ListNode fast = head, slow = head;

while (fast != null && fast.next != null) {

fast = fast.next.next;

slow = slow.next;

}

if (fast != null) { // odd nodes: let right half smaller

slow = slow.next;

}

slow = reverse(slow);

fast = head;

while (slow != null) {

if (fast.val != slow.val) {

return false;

}

fast = fast.next;

slow = slow.next;

}

return true;

}

public ListNode reverse(ListNode head) {

ListNode prev = null;

while (head != null) {

ListNode next = head.next;

head.next = prev;

prev = head;

head = next;

}

return prev;

}

}