int i = 0;

public int calculate(String s) {

Stack<Integer> stack = new Stack<>();

char operator = '+';

int n = 0;

while(i < s.length()){

char c = s.charAt(i);

i++;

if(Character.isDigit(c)) n = n * 10 + (c - '0');

if(c == '(') n = calculate(s);

if(i >= s.length() || c == '+' || c == '-' || c == '*' || c == '/' || c == ')'){

if(operator == '+') stack.add(n);

if(operator == '-') stack.add(-n);

if(operator == '*') stack.add(stack.pop() * n);

if(operator == '/') stack.add(stack.pop() / n);

operator = c;

n = 0;

}

if(c == ')') break;

}

return stack.stream().mapToInt(x -> x).sum();

}