[Leetcode][python]Rotate List/旋转链表

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def rotateRight(self, head, k):
        """
        :type head: ListNode
        :type k: int
        :rtype: ListNode
        """
        if k == 0:
            return head
        if head == None:
            return head
        dummy = ListNode(0)
        dummy.next = head
        p = dummy

        count = 0
        # 统计链表长度（这里实际上是从0开始数，数到4就结束，正好5个。）
        while p.next:  
            p = p.next
            count += 1
        step = count - ( k % count )

        # 此时p是最后一个数5，p.next指向null
        # p.next指向第一个数,这样p就是0,1,2,3,4,5,1,2,3,4,5
        p.next = dummy.next  
        for i in range(0, step):
            p = p.next

        # 此时p是最后一个未翻转的数3，将其下一个数4，放到head节点
        head = p.next
        # 此时head是4,5,1,2,3,4,5,1,2,3,4,5
        p.next = None
        # 此时head是4,5,1,2,3
        return head