[LeetCode]017-Letter Combinations of a Phone Number

タイトル:Given a digit string,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.
Input:Digit string “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.
Solution:ツリーの遍歴アルゴリズムを利用して,再帰的に重ねる.

vector<string> letterCombinations(string digits)
    {
        vector<string> result;
        vector<string> t_result;
        string temp = "";
        if(digits.size() == 0)
            return result;
        vector<char> tra_letters = getLetter(digits[0]);    
        if(digits.size() == 1)
        {
            for(int i =0;i<tra_letters.size();i++)
            {
                temp = tra_letters[i];
                result.push_back(temp);
            }
            return result;
        }

        for(int i =0 ;i<tra_letters.size();i++)
        {
            t_result = letterCombinations(digits.substr(1));
            for(int j=0;j<t_result.size();j++)
            {
                 temp = tra_letters[i] + t_result[j];
                 result.push_back(temp);
            }
        }
        return result;
    }
    vector<char> getLetter(char str)
    {
        vector<char> letters;
        switch(str)
        {
            case '2':
            {
                letters.push_back('a');
                letters.push_back('b');
                letters.push_back('c');
                break;
            }
            case '3':
            {
                letters.push_back('d');
                letters.push_back('e');
                letters.push_back('f');
                break;
            }
            case '4':
            {
                letters.push_back('g');
                letters.push_back('h');
                letters.push_back('i');
                break;
            }
            case '5':
            {
                letters.push_back('j');
                letters.push_back('k');
                letters.push_back('l');
                break;
            }
            case '6':
            {
                letters.push_back('m');
                letters.push_back('n');
                letters.push_back('o');
                break;
            }
            case '7':
            {
                letters.push_back('p');
                letters.push_back('q');
                letters.push_back('r');
                letters.push_back('s');
                break;
            }
            case '8':
            {
                letters.push_back('t');
                letters.push_back('u');
                letters.push_back('v');
                break;
            }
            case '9':
            {
                letters.push_back('w');
                letters.push_back('x');
                letters.push_back('y');
                letters.push_back('z');
                break;
            }
            default:return letters;
        }
        return letters;
    }