Given a reference to a linked list, return whether or not it forms a palindrome.

LinkedListPalindrome.kt

/*
Given a reference to a linked list, return whether or not it forms a palindrome. 

Ex: Given a reference to the following linked list...

head = 1->3->1, return true.
* */

fun isPalindrome(head: ListNode?): Boolean {
    var slow: ListNode? = head
    var fast: ListNode? = head
    //find the middle. By the time fast reaches zero, slow will be at half.
    while (fast?.next != null) {
        fast = fast.next?.next
        slow = slow?.next
    }
    // reverse second half
    var prev: ListNode? = null
    while (slow != null) {
        val temp = slow.next
        slow.next = prev
        prev = slow
        slow = temp
    }
    var left: ListNode? = head
    var right: ListNode? = prev
// compare two linked lists if they are equal
    while (right != null) {
        if (left?.`val` != right.`val`) {
            return false
        }
        left = left.next
        right = right.next
    }
    return true
}