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import java.util.Stack;

public class BasicCalculator {

public int calculate(String s) {

Stack<Integer> stack = new Stack<>();

int result = 0;

int number = 0;

int sign = 1;

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

char c = s.charAt(i);

if (Character.isDigit(c)) {

number = 10 * number + (int) (c - '0');

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

result += sign * number;

number = 0;

sign = 1;

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

result += sign * number;

number = 0;

sign = -1;

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

//we push the result first, then sign;

stack.push(result);

stack.push(sign);

//reset the sign and result for the value in the parenthesis

sign = 1;

result = 0;

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

result += sign * number;

number = 0;

result *= stack.pop(); //stack.pop() is the sign before the parenthesis

result += stack.pop(); //stack.pop() now is the result calculated before the parenthesis

}

}

if (number != 0) result += sign * number;

return result;

}

}