Problem description:

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

Example 1:

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Input: head = [1,2,3,4,5], k = 2
Output: [4,5,1,2,3]

Example 2:

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Input: head = [0,1,2], k = 4
Output: [2,0,1]

Solution:

Identify that k might be greater than list size. We need to % to get how many rotation we should do.

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# 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 not head or k == 0:
            return head
        size = 0
        dummy = ListNode(-1)
        dummy.next, p = head, dummy
        while p.next:
            p, size = p.next, size+1
        k %= size
        step, mid = size - k, dummy
        for _ in range(step):
            mid = mid.next
        p.next = dummy.next
        dummy.next = mid.next
        mid.next = None
        return dummy.next

time complexity: $O(n)$
space complexity: $O(1)$
reference:
related problem: