function isDigit (s: string): boolean {
    return ["0", "1", "2", "3", "4", "5", "6", "7", "8", "9"].includes(s);
}

function calculateSimpleInfix (queue: string[]): number {
    let result = 0;
    for (let i = 0; i < queue.length; i++) {
        const token = queue[i];
        if (token === "-") {
            result -= Number(queue[i + 1]);
            i += 1;
            continue;
        }
        if (token === "+") {
            continue;
        }
        result += Number (token);
    }
    return result;
}

function calculate(s: string): number {
    const stack = [];
    for (let i = 0; i < s.length; i++) {
        // Read token.
        let token = s[i];
        while (isDigit (s[i]) && isDigit (s[i + 1])) {
            i += 1;
            token += s[i];
        }
        if (token === " ") {
            continue;
        }

        if (token !== ")") {
            stack.push(token);
        }
        if (token === ")" || i === s.length - 1) {
            // evaluate expression until we meet an open bracket
            const evaluateQueue = [];
            while (true) {
                const simpleToken = stack.pop();
                if (simpleToken === "(" || simpleToken === undefined) {
                    break;
                }
                evaluateQueue.push(simpleToken);
            }
            evaluateQueue.reverse();
            stack.push(calculateSimpleInfix(evaluateQueue));
            continue;
        }
    }

    return Number(calculateSimpleInfix(stack));
};