LeetCode 17. Letter Combinations of a Phone Number高效解法

/************************************************************************
* 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.
*
************************************************************************/

vector<string> letterCombinations(string digits) {    vector<string> result;    if (digits.empty()) return vector<string>();    static const vector<string> v = { "", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz" };    result.push_back("");   // add a seed for the initial case    for (int i = 0; i < digits.size(); ++i) {        int num = digits[i] - '0';        if (num < 0 || num > 9) break;        const string& candidate = v[num];        if (candidate.empty()) continue;        vector<string> tmp;        for (int j = 0; j < candidate.size(); ++j) {            for (int k = 0; k < result.size(); ++k) {                tmp.push_back(result[k] + candidate[j]);            }        }        result.swap(tmp);    }    return result;}

//test

int main(int argc, char**argv){    string s="123";    vector<string> ss = letterCombinations(s);    printVector(ss);    return 0;}