code: def test_letter_combinations(self):
      digit1 = "23"
      answer1 = ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"]
      self.assertEqual(
          sorted(letter_combinations(digit1)), sorted(answer1)
      )
  
      digit2 = "34"
      answer2 = ["dg", "dh", "di", "eg", "eh", "ei", "fg", "fh", "fi"]
      self.assertEqual(
          sorted(letter_combinations(digit2)), sorted(answer2)
      )
  
imports: from algorithms.backtrack import add_operators, permute_iter, anagram, array_sum_combinations, unique_array_sum_combinations, combination_sum, get_factors, recursive_get_factors, find_words, generate_abbreviations, generate_parenthesis_v1, generate_parenthesis_v2, letter_combinations, palindromic_substrings, pattern_match, permute_unique, permute, permute_recursive, subsets_unique, subsets, subsets_v2
  import unittest
label_desc: Use image node_id 1 for calling the TestLetterCombinations obj's underlying member method code with example usage: obj.test_letter_combinations() without return types