[LeetCode] Letter Combinations of a Phone Number

其实我一开始读不懂题目。。我以为应该是出来的答案应该都是两位的，我还特地拷贝AC代码运行一下才知道题目意思= =虽然看了懂了题意还是不会做
代码挺容易懂的，运用递归，以level为digits的遍历指标，在遍历每个指标对应的text组成out,总的来说是DFS的运用

class Solution {
public:
  vector<string> letterCombinations(string digits) {
    string text[] = { "abc","def","ghi","jkl","mno","pqrs","tuv","wxyz" };
    vector<string>ans;
    if (digits.empty())return ans;
    string out = "";
    letterConmbinationsDFS(digits, text, 0, out, ans);
    return ans;
  }
  void letterConmbinationsDFS(string digits, string text[], int level, string out,
    vector<string>& ans) {
    if (level == digits.size())ans.push_back(out);
    else {
      string str = text[digits[level] - '2'];
      for (int i = 0; i < str.size(); ++i) {
        out.push_back(str[i]);
        letterConmbinationsDFS(digits, text, level + 1, out, ans);
        out.pop_back();
      }
    }
  }
};