letter_combinations_of_a_phone_number.rb

# https://leetcode.com/problems/letter-combinations-of-a-phone-number/
#
# Given a digit string, return all possible letter combinations that the
# number could represent.
#
# A mapping of digit to letters (just like on the telephone buttons) is given
# below:
#
#     1()     2(abc)  3(def)
#     4(ghi)  5(jkl)  6(mno)
#     7(pqrs) 8(tuv)  9(wxyz)
#     *()     0()     ^()
#
#    Input: Digit string "23"
#   Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
#
# Note: Although the above answer is in lexicographical order, your answer
# could be in any order you want.


# @param {String} digits
# @return {String[]}
def letter_combinations(digits)
  m = {
    '1' => '',     '2' => 'abc', '3' => 'def',
    '4' => 'ghi',  '5' => 'jkl', '6' => 'mno',
    '7' => 'pqrs', '8' => 'tuv', '9' => 'wxyz',
                   '0' => ''
  }

  return [] if digits.size == 0
  return m[digits[0]].chars if digits.size == 1

  letters = digits.chars.map { |digit| m[digit] }
  letters.map(&:chars).reduce(&:product).map(&:join)
end