Rotate List

Question

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.

Tags

• Linked Lis

Thought

Similar to the problem Rotate Array. However, since we don't know the length of the linked list, we need to count it at first. Then we can link the last node and the head node to form a circle here. After that, disconnect two nodes according to the number k.

Code

# 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
        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