public int calculate(String s) {

if (s == null || s.length() == 0) return 0;

s = s.replaceAll(" ", "");

char[] chars = s.toCharArray();

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

Stack<Character> ops = new Stack<>();

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

if (chars[k] >= '0' && chars[k] <= '9') {

StringBuilder sb = new StringBuilder();

while (k < chars.length && chars[k] >= '0' && chars[k] <= '9') {

sb.append(chars[k]);

k++;

}

value.add(Integer.parseInt(sb.toString()));

k--;

} else {

if (chars[k] == '(') {

ops.push(chars[k]);

} else if (chars[k] != ')') {

while (!ops.isEmpty() && ops.peek() != '(') {

value.push(calculate(value.pop(), value.pop(), ops.pop()));

}

ops.push(chars[k]);

} else {

while (ops.peek() != '(') {

value.push(calculate(value.pop(), value.pop(), ops.pop()));

}

ops.pop();

}

}

}

while (!ops.isEmpty()) {

value.push(calculate(value.pop(), value.pop(), ops.pop()));

}

return value.pop();

}

private int calculate(int v1, int v2, char op) {

if (op == '+') {

return v1 + v2;

} else {

return v2 - v1;

}

}

}