leetcode 17. Letter Combinations of a Phone Number

给出两个数字，看它在手机的键盘上对应多种做字母组合。这用到回溯法。

 var dict = ["", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"];// 0与1都没有字母
    function letterCombinations(digits) { //23
      var result = [], condidate = []
      if (!Object(digits).length) {
        return result;
      }

      function backtrack(start) {
        if (start == digits.length) { //到最后一个数字了
          result.push(condidate.join(''));
        } else {
          var str = dict[digits[start]];//2，3对应的字母
          for (var i = 0; i < str.length; i++) {
            condidate.push(str[i]);
            backtrack(start + 1);
            condidate.pop();
          }
        }
      }
      backtrack(0);
      return result;
    }