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-negativeintegers 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) {
        if (s == null || s.length() == 0) return 0;
        s = s.trim();
        s = s.replaceAll("\\s", "");

        int n = s.length();
        int ans = 0, sign = 1;

        Stack<Integer> stack = new Stack<>();
        for (int i = 0; i < n; i++){
            char c = s.charAt(i);

            if (Character.isDigit(c)){
                int cur = c - '0';
                while (i + 1 < n && Character.isDigit(s.charAt(i + 1))){
                    cur = cur * 10 + (s.charAt(i + 1) - '0');
                    i++;
                }

                ans += cur * sign;
            }
            else if (c == '+'){
                sign = 1;
            }
            else if (c == '-'){
                sign = -1;
            }
            else if (c == '('){
                stack.push(ans);
                stack.push(sign);
                ans = 0;
                sign = 1;
            }
            else if (c == ')'){
                ans = ans * stack.pop() + stack.pop();
            }

            //System.out.println("i: " + i + " ans: " + ans);
        }

        return ans;
    }
}