public:

vector<string> letterCombinations(string digits) {

vector<string> res;

unordered_map<char, vector<char>> m;

m['0'] = vector<char>();

m['1'] = vector<char>();

char charsFor2[] = {'a', 'b', 'c'};

m['2'] = vector<char>(charsFor2, charsFor2 + 3);

char charsFor3[] = {'d', 'e', 'f'};

m['3'] = vector<char>(charsFor3, charsFor3+ 3);

char charsFor4[] = {'g', 'h', 'i'};

m['4'] = vector<char>(charsFor4, charsFor4 + 3);

char charsFor5[] = {'j', 'k', 'l'};

m['5'] = vector<char>(charsFor5, charsFor5 + 3);

char charsFor6[] = {'m', 'n', 'o'};

m['6'] = vector<char>(charsFor6, charsFor6 + 3);

char charsFor7[] = {'p', 'q', 'r', 's'};

m['7'] = vector<char>(charsFor7, charsFor7 + 4);

char charsFor8[] = {'t', 'u', 'v'};

m['8'] = vector<char>(charsFor8, charsFor8 + 3);

char charsFor9[] = {'w', 'x', 'y', 'z'};

m['9'] = vector<char>(charsFor9, charsFor9 + 4);

res.push_back("");

for (int i = 0; i < digits.size(); i++) {

char d = digits[i];

vector<char> chars = m[d];

int n = res.size();

for (int j = 0; j < n; j++) {

string s = res.front();

for (int k = 0; k < chars.size(); k++) {

s.push_back(chars[k]);

res.push_back(s);

s.pop_back();

}

res.erase(res.begin()); // pop front

}

}

return res;

}