Letter Combinations of a Phone Number

Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order.

A mapping of digits to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.

Input: digits = "23"
Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]

const letterCombinations = function (digits) {
  if (!digits) return []

  const map = {
    2: 'abc',
    3: 'def',
    4: 'ghi',
    5: 'jkl',
    6: 'mno',
    7: 'pqrs',
    8: 'tuv',
    9: 'wxyz',
  }

  // We add all 3 or 4 characters of a given digit
  // to the "so-far" calculated result, one by one.
  //
  // Consider the so-far result as ["a", "b", "c"].
  // The given digit is 4, so its characters are: "g", "h", "i"
  // So we add each character to each element of the result.
  // ["ag", "bg", "cg"]

  let ans = ['']
  for (let i = 0; i < digits.length; i++) {
    const temp = []
    const curDigit = digits[i]
    const letters = map[curDigit]

    for (const l of letters) {
      for (const a of ans) {
        temp.push(a + l)
      }
    }

    ans = temp
  }

  return ans
}

const letterCombinations = function (digits) {
  if (!digits) return []

  const map = {
    2: 'abc',
    3: 'def',
    4: 'ghi',
    5: 'jkl',
    6: 'mno',
    7: 'pqrs',
    8: 'tuv',
    9: 'wxyz',
  }

  // For a given digits: 23, the pressed will look like this
  // ["abc", "def"]
  // Now, go like this:
  // a -> def
  // ad, ae, af
  // b -> def
  // bd, be, bf
  // c -> def
  // cd, ce, cf

  const pressed = []
  for (const d of digits) {
    pressed.push(map[d])
  }

  let ans = []
  function iterator(start, str) {
    if (start >= pressed.length) {
      ans.push(str)
      return
    }

    const curStr = pressed[start]
    for (let idx = 0; idx < curStr.length; idx++) {
      const curChar = curStr[idx]
      iterator(start + 1, str + curChar)
    }
  }

  iterator(0, '')
  return ans
}