Letter combinations Problem

letter_combinations.rb

# Problem: https://leetcode.com/problems/letter-combinations-of-a-phone-number/
# Given a digit string, return all possible letter combinations that the number could represent.
# Input:Digit string "23"
# Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].


@look_up = {
    "2" => "abc",
    "3" => "def",
    "4" => "ghi",
    "5" => "jkl",
    "6" => "mno",
    "7" => "pqrs",
    "8" => "tuv",
    "9" => "wxyz"
}

def get_letter_combinations(num_string)
  num_string.gsub!(/[01]/, "")
  if num_string.size == 1
    return @look_up[num_string].chars
  end
  result = []
  combinations = get_letter_combinations(num_string[1..-1])
  @look_up[num_string[0]].chars.each do |c|
    combinations.each do |cb|
      result << (c + cb)
    end
  end
  result
end