LeetCode-in-All

24. Swap Nodes in Pairs

Medium

Given a linked list, swap every two adjacent nodes and return its head. You must solve the problem without modifying the values in the list’s nodes (i.e., only nodes themselves may be changed.)

Example 1:

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

Output: [2,1,4,3]

Example 2:

Input: head = []

Output: []

Example 3:

Input: head = [1]

Output: [1]

Constraints:

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 swap_pairs(head :: ListNode.t() | nil) :: ListNode.t() | nil
  def swap_pairs(nil), do: nil
  def swap_pairs(head), do: swap_pairs(head, head.next)

  def swap_pairs(head, nil), do: head

  def swap_pairs(first, second) do
    %ListNode{
      second
      | next: %ListNode{
          first
          | next:
              unless second.next == nil do
                swap_pairs(second.next, second.next.next)
              else
                nil
              end
        }
    }
  end
end