find all permutations of characters in given string

string_permutations.rb

def permutations(s)
  return [s] if s.length == 1

  chars = s.chars.to_a

  chars.each_with_object([]) do |ch, ps|
    rest = chars.reject { |c| c.equal? ch }.join
    permutations(rest).each do |perm|
      ps << (ch + perm)
    end
  end
end

p permutations('abc')
# >> ["abc", "acb", "bac", "bca", "cab", "cba"]