golang力扣leetcode 17.电话号码的字母组合

func letterCombinations(digits string) []string {
  if digits == "" {
    return nil
  }
  mp := map[string]string{
    "2": "abc",
    "3": "def",
    "4": "ghi",
    "5": "jkl",
    "6": "mno",
    "7": "pqrs",
    "8": "tuv",
    "9": "wxyz",
  }
  n := len(digits)
  result := make([]string, 0)
  var dfs func(digits string, cur string)
  dfs = func(digits string, cur string) {
    if len(cur) == n {
      result = append(result, cur)
      return
    }
    ch := string(digits[0])
    cnt := mp[ch]
    for _, letter := range cnt {
      dfs(digits[1:], cur+string(letter))
    }
  }
  dfs(digits, "")
  return result
}