17. 电话号码的字母组合

/**
 * @param {string} digits
 * @return {string[]}
 */
var letterCombinations = function (digits) {
  function _compose(str1, str2) {
    let result = []
    if (str1.length === 0) return str2
    if (str2.length === 0) return str1
    for (let i = 0; i < str1.length; i++) {
      for (let j = 0; j < str2.length; j++) {
        result.push(str1[i] + str2[j])
      }
    }
    return result
  }
  const map = [, , 'abc', 'def', 'ghi', 'jkl', 'mno', 'pqrs', 'tuv', 'wxyz']
  if (digits.length === 0) return []
  if (digits.length === 1) return map[digits].split('')
  return digits
    .split('')
    .map((d) => map[d])
    .reduce(_compose, [])
}