main.exs

defmodule MergeSort do
  def sort(list) do
    do_sort(list, 0, length(list))
  end

  defp do_sort(list, left, right) when right - left == 1 do
    Enum.slice(list, left..(right - 1))
  end

  defp do_sort(list, left, right) do
    mid = floor((left + right) / 2)

    (do_sort(list, left, mid) ++ (do_sort(list, mid, right) |> Enum.reverse()))
    |> merge([])
  end

  defp merge([], acc), do: acc
  defp merge([num], acc), do: acc ++ [num]

  defp merge(list, acc) do
    {left, left_rest} = List.pop_at(list, 0)
    {right, right_rest} = List.pop_at(list, -1)

    if left <= right do
      merge(left_rest, acc ++ [left])
    else
      merge(right_rest, acc ++ [right])
    end
  end
end

MergeSort.sort([1, 3, 2, 5, 4, 11, 3, 21, 34, 2]) |> IO.inspect()

main.go

package main

import "fmt"

func main() {
	a := []int{1, 3, 2, 5, 4, 11, 3, 21, 34, 2}
	mergeSort(a)
	fmt.Println(a)
}

func mergeSort(a []int) {
	_mergeSort(a, 0, len(a))
}

func _mergeSort(a []int, left, right int) {
	if (right - left) == 1 {
		return
	}
	mid := (left + right) / 2
	_mergeSort(a, left, mid)
	_mergeSort(a, mid, right)

	buf := []int{}

	for i := left; i < mid; i++ {
		buf = append(buf, a[i])
	}
	for i := right - 1; i >= mid; i-- {
		buf = append(buf, a[i])
	}

	var (
		li = 0
		ri = len(buf) - 1
	)
	for i := left; i < right; i++ {
		if buf[li] <= buf[ri] {
			a[i] = buf[li]
			li++
		} else {
			a[i] = buf[ri]
			ri--
		}
	}
}