vector<string> commonChars(vector<string>& words) {

unordered_map<char, int> min_freq;

for (int i = 0; i < words[0].size(); i++) {

min_freq[words[0][i]]++;

}

for (int i = 1; i < words.size(); i++) {

unordered_map<char, int> freq;

for (int j = 0; j < words[i].size(); j++) {

freq[words[i][j]]++;

}

for (unordered_map<char, int>::iterator it = min_freq.begin();

it != min_freq.end(); ++it) {

char key = it->first;

int& value = it->second;

value = min(value, freq[key]);

}

}

vector<string> answer;

for (unordered_map<char, int>::iterator it = min_freq.begin();

it != min_freq.end(); ++it) {

char key = it->first;

int value = it->second;

while (value > 0) { // do value times

answer.push_back(string(1, key));

value--;

}

}

return answer;

}