17. 电话号码的字母组合（算法200题-40）

var letterCombinations = function (digits) {
  if (!digits) return []
  const map = new Map([
    ["1", ""],
    ["2", "abc"],
    ["3", "def"],
    ["4", "ghi"],
    ["5", "jkl"],
    ["6", "mno"],
    ["7", "pqrs"],
    ["8", "tuv"],
    ["9", "wxyz"],
  ])
  const list = []
  backtrack(list, "", 0)
  return list
  function backtrack(list, temp, num) {
    if (num === digits.length) {
      list.push(temp)
      return
    }
    const cur = map.get(digits[num])
    for (let i = 0; i < cur.length; i++) {
      temp += cur[i]
      backtrack(list, temp, num + 1)
      temp = temp.slice(0, -1)
    }
  }
};