0206-reverse-linked-list.rb

# frozen_string_literal: true

# 206. Reverse Linked List
# https://leetcode.com/problems/reverse-linked-list

=begin

Given the head of a singly linked list, reverse the list, and return the reversed list.

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

### Example 2:
Input: head = [1,2]
Output: [2,1]

### Example 3:
Input: head = []
Output: []

### Constraints:
* The number of nodes in the list is the range [0, 5000].
* -5000 <= Node.val <= 5000

=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
# @return {ListNode}
def reverse_list(head, prev = nil)
  while head
    head, head.next, prev = head.next, prev, head
  end
  prev
end