0025-reverse-nodes-in-k-group.rb

# frozen_string_literal: true

# 25. Reverse Nodes in k-Group
# https://leetcode.com/problems/reverse-nodes-in-k-group
# Hard

=begin
Given the head of a linked list, reverse the nodes of the list k at a time, and return the modified list.

k is a positive integer and is less than or equal to the length of the linked list. If the number of nodes is not a multiple of k then left-out nodes, in the end, should remain as it is.

You may not alter the values in the list's nodes, only nodes themselves may be changed.

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

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

Constraints:
* The number of nodes in the list is n.
* 1 <= k <= n <= 5000
* 0 <= Node.val <= 1000
=end

# Definition for singly-linked list.
# class ListNode
# attr_accessor :val, :next
# def initialize(val = 0, _next = nil)
# @val = val
# @next = _next
# end
# end
# @param {ListNode} head
# @param {Integer} k
# @return {ListNode}
defreverse_k_group(head,k)
dummy=ListNode.new("dummy",head)
prev=dummy

loopdo
scout=prev

k.timesdo
breakunlessscout
scout=scout.next
end

breakunlessscout

pivot=prev.next

untilprev.next == scout
prev.next,pivot.next.next=pivot.next.next,prev.next
prev.next,pivot.next=pivot.next,prev.next
end

k.times{prev=prev.next}
end

dummy.next
end