/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
class Solution {
public boolean isPalindrome(ListNode head) {
if(head == null){
return true;
}
ListNode firstHalfEnd = endOfFirstHalf(head);
ListNode secondHalfStart = reverseList(firstHalfEnd.next);
ListNode p1 = head;
ListNode p2 = secondHalfStart;
boolean result = true;
while(result && p2 != null){
if(p1.val != p2.val){
result = false;
}
p1 = p1.next;
p2 = p2.next;
}
firstHalfEnd.next = reverseList(secondHalfStart);
return result;
}
private ListNode reverseList(ListNode head){
ListNode prev = null;
ListNode curr = head;
while(curr != null){
ListNode nextTemp = curr.next;
curr.next = prev;
prev = curr;
curr = nextTemp;
}
return prev;
}
private ListNode endOfFirstHalf(ListNode head){
ListNode fast = head;
ListNode slow = head;
while(fast.next != null && fast.next.next != null){
fast = fast.next.next;
slow = slow.next;
}
return slow;
}
}