Given a linked list, rotate the list to the right by k places, where k is non-negative.
遍历链表，计算链表长度 l，并把链表变成环，然后从 l-k 出断开即可获取旋转后的链表。
# Definition for singly-linked list.
class ListNode:
def __init__(self, x):
self.val = x
self.next = None
class Solution:
def rotateRight(self, head: ListNode, k: int) -> ListNode:
if not head:
return head
length = 1
cur = head
while cur.next:
length += 1
cur = cur.next
cur.next = head
for i in range(length - k % length - 1):
head = head.next
new_head = head.next
head.next = None
return new_head