bool isValid(string s) {

stack<char> bracket;

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

if (s[i] == '(' || s[i] == '{' || s[i] == '[') {

bracket.push(s[i]);

} else {

if (bracket.empty() || (s[i] == ')' && bracket.top() != '(') ||

(s[i] == '}' && bracket.top() != '{') ||

(s[i] == ']' && bracket.top() != '[')) {

return false;

}

bracket.pop();

}

}

return bracket.empty();

}