[LeetCode] #17 Letter Combinations of a Phone Number

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"].

 本文是简单的词语匹配问题，先用letter存放字母键对应的字母组，然后就是两个字符的组合问题。时间：2ms

代码如下：

class Solution {
public:
    string getString(char s){
        switch (s){
        case '0':return ""; break;
        case '2':return "abc"; break;
        case '3':return "def"; break;
        case '4':return "ghi"; break;
        case '5':return "jkl"; break;
        case '6':return "mno"; break;
        case '7':return "pqrs"; break;
        case '8':return "tuv"; break;
        case '9':return "wxyz"; break;
        default:return ""; break;
        }
    }
    vector<string> letterCombinations(string digits) {
        vector<string> letter;
        int n = letter.size();
        string strf = getString(digits[0]);
        for (int i = 0; i < strf.size(); i++){
            string s = "";
            s += strf[i];
            letter.push_back(s);
        }
        for (int i = 1; i < digits.size(); i++){
            string strl = getString(digits[i]);
            n = letter.size();
            for (int j = 0; j < n; j++){
                strf = letter[j];
                for (int z = 0; z < strl.size(); z++){
                    letter.push_back(strf + strl[z]);
                }
            }
            letter.erase(letter.begin(), letter.begin() + n);
        }
        return letter;
    }
};