Basic Calculator

Implement a basic calculator to evaluate a simple expression string.

The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces .

Example 1:

Input: "1 + 1"
Output: 2

Example 2:

Input: " 2-1 + 2 "
Output: 3

Example 3:

Input: "(1+(4+5+2)-3)+(6+8)"
Output: 23

Note:

• You may assume that the given expression is always valid.

• Do not use the eval built-in library function.

class Solution {
    public int calculate(String s) {
        s = '(' + s + ')';
        Stack<String> st = new Stack<>();
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == '(')
                st.push("(");
            else if (s.charAt(i) == ')') {
                int res = 0;
                while (st.size() != 0) {
                    int x = Integer.parseInt(st.pop());
                    String second = st.pop();
                    if (second.compareTo("+") == 0)
                        res += x;
                    else if (second.compareTo("-") == 0)
                        res -= x;
                    else if (second.compareTo("(") == 0) {
                        res += x;
                        break;
                    }
                }
                st.push(Integer.toString(res));
            } else if (s.charAt(i) >= '0' && s.charAt(i) <= '9') {
                StringBuilder str = new StringBuilder();
                while (i < s.length() && s.charAt(i) >= '0' && s.charAt(i) <= '9')
                    str.append(s.charAt(i++));
                i--;
                st.push(str.toString());
            } else if (s.charAt(i) == '-' || s.charAt(i) == '+')
                st.push(s.charAt(i) + "");
        }
        return Integer.parseInt(st.pop());
    }
}