Leetcode61

!!Given the head of a linked list, rotate the list to the right by k places.!!

 

# Definition for singly-linked list.

# class ListNode(object):

#     def __init__(self, val=0, next=None):

#         self.val = val

#         self.next = next

class Solution(object):

    def rotateRight(self, head, k):

        """

        :type head: ListNode

        :type k: int

        :rtype: ListNode

        """

        if head == None:

            return []

        count = 1

        temp = head

        while (temp.next != None):

            count +=1

            temp = temp.next

            # count = length, so the length remaining same seq is count - k -1

 

        if k == 0:return head;

        #else

        head = temp.next

        temp.next = None

        while(count -k -1): #ensure the amount of reversed nodes

            #to reverse

            head = temp.next

            temp.next = None #clearance

            return head

 

A better way:

#先把链表首尾相连，再找到位置断开循环

class Solution(object):
    def rotateRight(self, head, k):
        if head is None or head.next is None: return head
        start, end, len = head, None, 0
        while head:
            end = head
            head = head.next
            len += 1
        end.next = start
        pos = len - k % len
        while pos > 1:
            start = start.next
            pos -= 1
        ret = start.next
        start.next = None
        return ret