回溯与递归

1. map<char, string>my_map;
    void backTrack(vector<string>&combinations,string digits, int index,string &combination) {
       if (index == digits.size()) {
           combinations.push_back(combination);
       }
       string letters = my_map[digits[index]];
       for (int i = 0; i < letters.size(); i++) {
           combination.push_back(letters[i]);
           backTrack(combinations, digits, index + 1, combination);
           combination.pop_back();
       }
   }
   vector<string> letterCombinations(string digits) {
       my_map.insert(pair<char, string>('2', "abc"));
       my_map.insert(pair<char, string>('3', "def"));
       my_map.insert(pair<char, string>('4', "ghi"));
       my_map.insert(pair<char, string>('5', "jkl"));
       my_map.insert(pair<char, string>('6', "mno"));
       my_map.insert(pair<char, string>('7', "pqrs"));
       my_map.insert(pair<char, string>('8', "tuv"));
       my_map.insert(pair<char, string>('9', "wxyz"));
       vector<string>combinations;
       if (digits.empty()) {
           return combinations;
       }
       string combination;
       backTrack(combinations,digits, 0, combination);
       for (int i = 0; i < combinations.size(); i++) {
           cout << combinations[i] << endl;
       }
       return combinations;
   }