BasicCalculator.java

import java.util.*;

class BasicCalculator {
    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        String str = s.nextLine();
        System.out.println(calculate(str));
    }

    public static int calculate(String s) {
        if(s == null) return 0;
        
        int result = 0;
        int sign = 1;
        int num = 0;

        Stack<Integer> stack = new Stack<Integer>();
        stack.push(sign);

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

            if(c >= '0' && c <= '9') {
                num = num * 10 + (c - '0');

            } else if(c == '+' || c == '-') {
                result += sign * num;
                sign = stack.peek() * (c == '+' ? 1: -1); 
                num = 0;

            } else if(c == '(') {
                stack.push(sign);

            } else if(c == ')') {
                stack.pop();
            }
        }

        result += sign * num;
        return result;
    }
    public int calculateEditoral(String s) {

        Stack<Integer> stack = new Stack<Integer>();
        int operand = 0;
        int result = 0; // For the on-going result
        int sign = 1;  // 1 means positive, -1 means negative

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

            char ch = s.charAt(i);
            if (Character.isDigit(ch)) {

                // Forming operand, since it could be more than one digit
                operand = 10 * operand + (int) (ch - '0');

            } else if (ch == '+') {

                // Evaluate the expression to the left,
                // with result, sign, operand
                result += sign * operand;

                // Save the recently encountered '+' sign
                sign = 1;

                // Reset operand
                operand = 0;

            } else if (ch == '-') {

                result += sign * operand;
                sign = -1;
                operand = 0;

            } else if (ch == '(') {

                // Push the result and sign on to the stack, for later
                // We push the result first, then sign
                stack.push(result);
                stack.push(sign);

                // Reset operand and result, as if new evaluation begins for the new sub-expression
                sign = 1;
                result = 0;

            } else if (ch == ')') {

                // Evaluate the expression to the left
                // with result, sign and operand
                result += sign * operand;

                // ')' marks end of expression within a set of parenthesis
                // Its result is multiplied with sign on top of stack
                // as stack.pop() is the sign before the parenthesis
                result *= stack.pop();

                // Then add to the next operand on the top.
                // as stack.pop() is the result calculated before this parenthesis
                // (operand on stack) + (sign on stack * (result from parenthesis))
                result += stack.pop();

                // Reset the operand
                operand = 0;
            }
        }
        return result + (sign * operand);
    }

}