Given a string having digits from 1 – 9 inclusive, return all the possible letter combinations that can be made from the numbers in the string. Return the answer in any order.

1-letter-combinations-number.js

const lettersMap = {
  1: [" "],
  2: ["a", "b", "c"],
  3: ["d", "e", "f"],
  4: ["g", "h", "i"],
  5: ["j", "k", "l"],
  6: ["m", "n", "o"],
  7: ["p", "q", "r", "s"],
  8: ["t", "u", "v"],
  9: ["w", "x", "y", "z"],
};

function letterCombinationsRec(digits, index, combination, results) {
  // base case - combination length equals digit length
  // also at this point index points outside of digits, so there are no more digits left to combine
  if (combination.length === digits.length) {
    results.push(combination.trim());
    return;
  }
  // take letters corresponding to digit at position index
  const letters = lettersMap[digits[index]];
  // combine each letter with current combination,
  // and pass it further incrementing index by 1
  // so subsequent recursive calls can combine a new combination with the rest of the digits
  for (let i = 0; i < letters.length; ++i) {
    letterCombinationsRec(
      digits,
      index + 1,
      `${combination}${letters[i]}`,
      results
    );
  }
}

export function letterCombinations(digits) {
  const results = [];
  letterCombinationsRec(digits, 0, "", results);
  return results;
}

2-README.md

// note: the space complexity is different since this solution uses a string instead of array to contain a combination