BasicCalculator.java

class Solution {
    // TC : O(n)
    public static int calculate(String s) {
        int len = s.length();
        int sign = 1;
        int ans = 0;
        int currNo = 0;
        Stack<Integer> stack = new Stack<Integer>();
        for (int i = 0; i < len; i++) {
            if (Character.isDigit(s.charAt(i))) {
                currNo = s.charAt(i) - '0';
                while (i + 1 < len && Character.isDigit(s.charAt(i + 1))) {
                    currNo = currNo * 10 + s.charAt(i + 1) - '0';
                    i++;
                }
                currNo = currNo * sign;
                ans += currNo;
                currNo = 0;
                sign = 1;
            } else if (s.charAt(i) == '+')
                sign = 1;
            else if (s.charAt(i) == '-')
                sign = -1; //  -1 respresents negative sign
            else if (s.charAt(i) == '(') {
                stack.push(ans); // store the result calculated so far
                stack.push(sign); // store the upcoming sign
                ans = 0;
                sign = 1;
            } else if (s.charAt(i) == ')') {
                int prevSign = stack.pop();
                ans = prevSign* ans ;
                int prevAns = stack.pop();
                ans = ans + prevAns;
            }

        }
        return ans;
    }
}