17 - 电话号码的字母组合 about leetcode HOT 1 OPEN

function letterCombinations(digits: string): string[] {
  if (digits.length === 0) return [];
  const map = {
    2: "abc",
    3: "def",
    4: "ghi",
    5: "jkl",
    6: "mno",
    7: "pqrs",
    8: "tuv",
    9: "wxyz",
  };
  const result: string[] = [];
  const dfs = (digits: string, str: string = "") => {
    if (digits.length === 0) {
      result.push(str);
      return;
    }
    const digit = digits[0];
    const letters = map[digit];
    for (let i = 0; i < letters.length; i++) {
      dfs(digits.substr(1), str + letters[i]);
    }
  };
  dfs(digits);
  return result;
}