Problem

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.

Solution

class Solution {
    public int calculate(String s) {
        s = s.trim().replaceAll(" ", "");
        Stack<Integer> stack = new Stack<>();
        int sign = 1, res = 0;
        
        System.out.println(s);
        for (int i = 0; i < s.length(); i++) {
            char ch = s.charAt(i);
            if (ch == '+') {
                sign = 1;
            } else if (ch == '-') {
                sign = -1;
            } else if (ch == '(') {
                stack.push(res);
                stack.push(sign);
                res = 0;
                sign = 1;
            } else if (ch == ')') {
                res *= stack.pop();
                res += stack.pop();
            } else {
                int sum = 0;
                while (i < s.length() && Character.isDigit(s.charAt(i))) {
                    sum = sum*10 + (s.charAt(i)-'0');
                    i++;
                }
                res += sum*sign;
                i--;
            } 
        }
        return res;
    }
}