LeetCode-17. Letter Combinations of a Phone Number

Given a string containing digits from ​​2-9​​ inclusive, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.

Example:

Input: "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

Note:

Although the above answer is in lexicographical order, your answer could be in any order you want.

题解：

class Solution {
public:
  static void dfs(map<char, string> dic, string s, int n, vector<string> &ans, int now, string tmp) {
    for (int i = 0; i < dic[s[now]].length(); i++) {
      tmp += dic[s[now]][i];
      if (now == n - 1) {
        ans.push_back(tmp);
      }
      else {
        dfs(dic, s, n, ans, now + 1, tmp);
      }
      tmp.pop_back();
    }
  }
  vector<string> letterCombinations(string digits) {
    vector<string> ans;
    map<char, string> dic;
    string tmp;
    dic.insert(pair<char, string>('2', "abc"));
    dic.insert(pair<char, string>('3', "def"));
    dic.insert(pair<char, string>('4', "ghi"));
    dic.insert(pair<char, string>('5', "jkl"));
    dic.insert(pair<char, string>('6', "mno"));
    dic.insert(pair<char, string>('7', "pqrs"));
    dic.insert(pair<char, string>('8', "tuv"));
    dic.insert(pair<char, string>('9', "wxyz"));
    int n = digits.length();
    dfs(dic, digits, n, ans, 0, tmp);
    return ans;
  }
};