Rotate Right - LinkedList

var rotateRight = function(head, k) {

    if(head === null || head.next === null){
        return head;
    }
    for(let i=0; i<k; i++){
        let current = head;
        let previous = head;
        while (current?.next){
            previous = current;
            current = current.next;
        }
        let lastElement = previous.next;
        previous.next = null;
        lastElement.next = head;  
        head=lastElement
    }
    return head;
};

  let current = head;
    let slow = head;
    let fast = head;
    let size = 0;

    if(!head || !head.next) return head;

    while(current) {
        current = current.next;
        size++;
    }

    let shiftBy = k % size;
    if (shiftBy === 0) return head;


    while(shiftBy--) {
        fast = fast.next;
    }

    while(fast.next) {
        fast = fast.next;
        slow = slow.next;
    }


    fast.next = head;
    head = slow.next;
    slow.next = null;

    return head;