BasicCalculator.java

/*https://leetcode.com/problems/basic-calculator/*/

class Solution {
    public int calculate(String s) {
        Stack<Integer> st = new Stack<>();
        int sum = 0, sign = 1;
        for(int i = 0; i < s.length(); ++i)
        {
            char ch = s.charAt(i);

            //if the character is a digit, trace the entire number and store it along with its sign
            if (Character.isDigit(ch))
            {
                int val = 0;
                while(i < s.length() && Character.isDigit(s.charAt(i)))
                    val = val*10+(s.charAt(i++)-'0');
                --i;
                val = val*sign;
                sum += val;
                sign = 1;
            }

            //for open brackets, push the result and the sign to stack and reset them
            else if (ch == '(')
            {
                st.push(sum);
                st.push(sign);
                sum = 0;
                sign = 1;
            }

            //for closing brackets, pop the result and the sign from the stack
            else if (ch == ')')
            {
                sum *= st.pop();
                sum += st.pop();
            }

            //for unary -, change the sign
            else if (ch == '-')
                sign *= -1;
        }
        return sum;
    }
}