solution.ts

class ListNode {
    val: number
    next: ListNode | null
    constructor (val?: number, next?: ListNode | null) {
        this.val = (val === undefined ? 0 : val);
        this.next = (next === undefined ? null : next);
    }
}

type ListNodeNull = ListNode|null;
function rotateRight (head: ListNode | null, k: number): ListNode | null {
    if (head === null) {
        return head;
    }
    let node:ListNodeNull = head;
    let count = 0;
    while (node !== null) {
        node = node.next;
        count++;
    }
    k %= count;
    if (k === 0) {
        return head;
    }
    let fast:ListNodeNull = head;
    while (k--) {
        fast = (<ListNode>fast).next;
    }
    let slow:ListNodeNull = head;
    while ((fast as ListNode).next !== null) {
        fast = (fast as ListNode).next;
        slow = (slow as ListNode).next;
    }
    const newHead = (<ListNode>slow).next;
    (<ListNode>slow).next = null;
    (<ListNode>fast).next = head;
    return newHead;
}