61. Rotate List 把最后一个节点插入头节点后

首先需要发现，k与链表长度N的关系。
其次，重复执行“把最后一个节点插入头节点后”
生成带头节点使得操作变得简单

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def rotateRight(self, head: ListNode, k: int) -> ListNode:
        if k == 0 or head is None or head.next is None:
            return head
        H = ListNode()
        H.next = head
        p = H
        length = 0
        while p.next:
            length += 1
            p = p.next
        k = k % length
            
        for i in range(k):
            p = H.next
            q = H
            while p.next:
                q = p
                p = p.next
            p.next = H.next
            H.next = p
            q.next = None
        return H.next