<p>Given a string <code>s</code> representing a valid expression, implement a basic calculator to evaluate it, and return <em>the result of the evaluation</em>.</p>

<p><strong>Note:</strong> You are <strong>not</strong> allowed to use any built-in function which evaluates strings as mathematical expressions, such as <code>eval()</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;1 + 1&quot;
<strong>Output:</strong> 2
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> s = &quot; 2-1 + 2 &quot;
<strong>Output:</strong> 3
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;(1+(4+5+2)-3)+(6+8)&quot;
<strong>Output:</strong> 23
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= s.length &lt;= 3 * 10<sup>5</sup></code></li>
	<li><code>s</code> consists of digits, <code>&#39;+&#39;</code>, <code>&#39;-&#39;</code>, <code>&#39;(&#39;</code>, <code>&#39;)&#39;</code>, and <code>&#39; &#39;</code>.</li>
	<li><code>s</code> represents a valid expression.</li>
	<li><code>&#39;+&#39;</code> is <strong>not</strong> used as a unary operation (i.e., <code>&quot;+1&quot;</code> and <code>&quot;+(2 + 3)&quot;</code> is invalid).</li>
	<li><code>&#39;-&#39;</code> could be used as a unary operation (i.e., <code>&quot;-1&quot;</code> and <code>&quot;-(2 + 3)&quot;</code> is valid).</li>
	<li>There will be no two consecutive operators in the input.</li>
	<li>Every number and running calculation will fit in a signed 32-bit integer.</li>
</ul>

class Calc {
public:
    Calc(string& source) {
        s = source;
        s.erase(std::remove(s.begin(), s.end(), ' '), s.end());
    }

    int eval() {
        return eval_expr();
    }

private:
    string s;
    size_t i{ 0 };

    int eval_expr() {
        int sum{ 0 };
        while (has_next()) {
            if (peek() == ')') {
                advance();
                break;
            }
            int term{ 0 };
            bool has_minus{ false };
            if (peek() == '-') {
                has_minus = true;
                advance();
            }
            if (peek() == '(') {
                advance();
                term = eval_expr();
            } else if (std::isdigit(peek())) {
                term = eval_number();
            } else if (peek() == '+') {
                advance();
                continue;
            }
            sum += has_minus ? -term : term;
        }
        return sum;
    }

    int eval_number() {
        std::string digits{};
        while (std::isdigit(peek())) {
            digits.push_back(peek());
            advance();
        }
        return std::stoi(digits);
    }

    bool has_next() {
        return i < s.length();
    }

    char peek() {
        return s[i];
    }

    void advance() {
        i++;
    }
};

class Solution {
public:
    int calculate(string s) {
        return Calc{ s }.eval();
    }
};
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