Ruby で順列

my_permutation.rb

def my_permutation(sequence, r = nil)
  r = sequence.size if r.nil? || sequence.size < r
  result = []
  enumerate(sequence, r, [], result)
  result
end

def enumerate(sequence, r, one_enumeration, result)
  if one_enumeration.size == r
    result << one_enumeration.dup
    return
  end

  sequence.each do |n|
    # 再帰時にはコピーを渡す
    dup_sequence = sequence.dup
    dup_one_enumeration = one_enumeration.dup

    # sequence から取り除いた要素を列挙用配列へ追加
    dup_sequence.delete(n)
    dup_one_enumeration << n

    enumerate(dup_sequence, r, dup_one_enumeration, result)

    # 列挙用配列へ追加した要素を取り除き sequence へ戻す
    dup_sequence << n
    dup_one_enumeration.delete(n)
  end
end

p my_permutation(['a', 'b', 'c'])
p my_permutation(['a', 'b', 'c', 'd'], 2)