[leetcode]Rotate List @ Python

原題地址：https://oj.leetcode.com/problems/rotate-list/

題意：

Given a list, rotate the list to the right by k places, where k is non-negative.

For example:
Given 1->2->3->4->5->NULL and k = 2,
return 4->5->1->2->3->NULL.

解題思路：循環右移一條鏈表，比如k=2，（1，2，3，4，5）循環右移兩位變為（4，5，1，2，3）。由於k值有可能比鏈表長度大很多，所以先要用一個count變量求出鏈表的長度。而k%count就是循環右移的步數。

代碼：

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

class Solution:
    # @param head, a ListNode
    # @param k, an integer
    # @return a ListNode
    def rotateRight(self, head, k):
        if k == 0:
            return head
        if head == None:
            return head
        dummy = ListNode(0)
        dummy.next = head
        p = dummy
        count = 0
        while p.next:
            p = p.next
            count += 1
        p.next = dummy.next
        step = count - ( k % count )
        for i in range(0, step):
            p = p.next
        head = p.next
        p.next = None
        return head