bool isLetter(char c) {

return c >= 'a' && c <= 'z';

}

string mostCommonWord(string paragraph, vector<string>& banned) {

unordered_map<string, int> banned_map, words_map;

for (auto w:banned) {

banned_map[w]++;

}

//conert the string to lower case.

transform(paragraph.begin(), paragraph.end(), paragraph.begin(), ::tolower);

//transfer the symbols to space.

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

if ( !isLetter(paragraph[i]) ){

paragraph[i] = ' ';

}

}

string word;

for(auto c:paragraph) {

if (isLetter(c)) {

word += c;

}else{

if ( word.size()>0 ) {

words_map[word]++;

}

word="";

}

}

if ( word.size()>0 ) words_map[word]++;

string result;

int max_cnt=0;

// go through the words_map

for (auto const& w : words_map) {

if ( banned_map.find(w.first) != banned_map.end() ) {

continue;

}

if (max_cnt < w.second) {

result = w.first;

max_cnt = w.second;

}

}

return result;

}