【LeetCode算法練習（C++）】Letter Combinations of a Phone Number

題目：
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”].

鏈接：Letter Combinations of a Phone Number

解法：暴力循環。時間O(n^m)

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> bot;
        vector<string> ans;
        vector<string> tmp;
        bot.push_back("");
        bot.push_back("");
        bot.push_back("abc");
        bot.push_back("def");
        bot.push_back("ghi");
        bot.push_back("jkl");
        bot.push_back("mno");
        bot.push_back("pqrs");
        bot.push_back("tuv");
        bot.push_back("wxyz");

        for (int i = 0; i < bot[digits[0] - '0'].size(); i++) {
            char c = bot[digits[0] - '0'][i];
            string str;
            stringstream stream;
            stream << c;
            str = stream.str();
            ans.push_back(str);
        }
        for (int i = 1; i < digits.length(); i++) {
            tmp = ans;
            ans.clear();
            for (int k = 0; k < tmp.size(); k++) {
                for (int j = 0; j < bot[digits[i] - '0'].size(); j++) {
                    ans.push_back(tmp[k] + bot[digits[i] - '0'][j]);
                }
            }
        }
        return ans;
    }
};

Runtime: 3 ms