public int calculate(String s) {

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

int num = 0;

char preOp = '+';

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

if (Character.isDigit(s.charAt(i))) {

num = num * 10 + (s.charAt(i) - '0');

}

if (i == s.length() - 1 || s.charAt(i) == '-' || s.charAt(i) == '+' || s.charAt(i) == '*' || s.charAt(i) == '/') {

if (preOp == '+') stack.add(num);

else if (preOp == '-') stack.add(-num);

else if (preOp == '*') stack.add(num * stack.pop());

else stack.add(stack.pop() / num);

num = 0;

preOp = s.charAt(i);

}

}

int res = 0;

while (!stack.isEmpty()) res += stack.pop();

return res;

}