[17] 电话号码的字母组合

/**
 * @param {string} digits
 * @return {string[]}
 */
var letterCombinations = function (digits) {
  if (digits === null || digits.length === 0) {
    return [];
  } else {
    return deep("", digits);
  }
};

const deep = function (start, str) {
  let ans = [];
  const arr = getStrArr(str[0]);

  if (str.length === 1) {
    for (let i = 0; i < arr.length; i++) {
      ans.push(start + arr[i]);
    }
  } else {
    for (let i = 0; i < arr.length; i++) {
      ans = ans.concat(deep(start + arr[i], str.substring(1)));
    }
  }
  return ans;
}

const getStrArr = function (c) {
  switch (c) {
    case '2':
      return ['a', 'b', 'c'];
    case '3':
      return ['d', 'e', 'f'];
    case '4':
      return ['g', 'h', 'i'];
    case '5':
      return ['j', 'k', 'l'];
    case '6':
      return ['m', 'n', 'o'];
    case '7':
      return ['p', 'q', 'r', 's'];
    case '8':
      return ['t', 'u', 'v'];
    case '9':
      return ['w', 'x', 'y', 'z'];
    default:
      return [];
  }
}