LeetCode-in-All

206. Reverse Linked List

Easy

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:

Follow up: A linked list can be reversed either iteratively or recursively. Could you implement both?

Solution

# Definition for singly-linked list.
#
# defmodule ListNode do
#   @type t :: %__MODULE__{
#           val: integer,
#           next: ListNode.t() | nil
#         }
#   defstruct val: 0, next: nil
# end

defmodule Solution do
  @spec reverse_list(head :: ListNode.t | nil) :: ListNode.t | nil
  def reverse_list(nil),                              do: nil
  def reverse_list(%ListNode{val: v, next: n}),       do: reverse_list(n, %ListNode{val: v, next: nil})
  def reverse_list(%ListNode{val: v, next: n}, acc),  do: reverse_list(n, %ListNode{val: v, next: acc})
  def reverse_list(nil, acc),                         do: acc
end