var isPalindrome = function(head) {

    const midElementOfList = getMiddleElemnt(head);
    const reversedFromMiddleTillEnd = reverseList(midElementOfList);
    return checkIfPalindrome(head, reversedFromMiddleTillEnd);
};

var getMiddleElemnt = (head) => {
let slow = head;
let fast = head;

    while(fast && fast.next !==null && fast.next.next !== null){
        slow = slow.next;
        fast= fast.next.next;
    }
    return slow;
}
var checkIfPalindrome = (head, reverseList) =>{
    while(head!==null && reverseList !==null){
        if(head.val!== reverseList.val){
            return false;
        }
        head = head.next;
        reverseList= reverseList.next;
    }
    return true;
}

var reverseList = function(head) {

const reverseR = (head) => {
   if (head === null || head.next === null) {
    return head;
  }
   let rest_trail = reverseR(head.next);
    const nextOfHead = head.next;
    nextOfHead.next = head;
    head.next = null;
    return rest_trail;
}
return reverseR(head);   
}