0234-palindrome-linked-list.java

//reverse the later half
//after reversing just start comparing if at any time the value doesn't match it's not a palindrome, i.e. return false, else it's a palindrome and return true.
//It'll automatically take care of the edge cases of odd and even

class Solution {

    public boolean isPalindrome(ListNode head) {
        ListNode fast = head;
        ListNode slow = head;
        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
        }
        ListNode temp = reverse(slow);
        while (temp != null && head != null) {
            if (temp.val != head.val) return false;
            temp = temp.next;
            head = head.next;
        }
        return true;
    }

    public ListNode reverse(ListNode head) {
        ListNode p = null;
        ListNode q = null;
        ListNode r = head;
        while (r != null) {
            p = q;
            q = r;
            r = r.next;
            q.next = p;
        }
        return q;
    }
}