LeetCode Top Interview 150

61. Rotate List

Medium

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

Example 1:

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

Output: [4,5,1,2,3]

Example 2:

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

Output: [2,0,1]

Constraints:

• The number of nodes in the list is in the range [0, 500].
• -100 <= Node.val <= 100
• 0 <= k <= 2 * 109

Solution

from typing import Optional

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

class Solution:
    def rotateRight(self, head: Optional[ListNode], k: int) -> Optional[ListNode]:
        if head is None or k == 0:
            return head
        tail = head
        # find the count and let tail points to last node
        count = 1
        while tail is not None and tail.next is not None:
            count += 1
            tail = tail.next
        # calculate number of times to rotate by count modulas
        times = k % count
        if times == 0:
            return head
        temp = head
        # iterate and go to the K+1 th node from the end or count - K - 1 node from start
        for i in range(1, count - times):
            if temp is not None:
                temp = temp.next
        new_head = None
        if temp is not None and tail is not None:
            new_head = temp.next
            temp.next = None
            tail.next = head
        return new_head

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