61. Rotate List

• categories: Code, Interview Question, leetcode, Medium
• source: https://leetcode.com/problems/rotate-list
• topics: Linked List

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

Idea

Find the length and last element, break the list at len-k and unite tail with head

# 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: Optional[ListNode], k: int) -> Optional[ListNode]:
        if not head or k == 0:
            return head
        
        list_len = 1
        cur = head
        while cur.next:
            list_len += 1
            cur = cur.next
        
        k = k % list_len
        last = cur
        if k == 0:
            return head
 
        cur = head
        for _ in range(list_len - k - 1):
            cur = cur.next
        last.next = head
        head = cur.next
        cur.next = None
        
        return head