17. 电话号码的字母组合

function letterCombinations(digits: string): string[] {
  if (digits.length === 0) {
    return [];
  }
 
  type mapKeys = keyof typeof map;
  const ans: string[] = [];
  const map = {
    "2": "abc",
    "3": "def",
    "4": "ghi",
    "5": "jkl",
    "6": "mno",
    "7": "pqrs",
    "8": "tuv",
    "9": "wxyz",
  };

  if (digits.length === 1) {
    return map[digits as mapKeys].split("");
  }

  //创建递归函数
  const handleRes = (curStr: string, i: number) => {
    if (i > digits.length - 1) {
      ans.push(curStr);
      return;
    }

    const letters = map[digits[i] as mapKeys];
    for (const letter of letters) {
      handleRes(curStr + letter, i + 1);
    }
  };

  //设置初始值，下标从0开始
  handleRes("", 0);
  return ans;
}