<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 Solution {
public:
    int calculate(const string& s) {
        int curRes = 0;

        int sign = 1;

        for (int i = 0; i < s.size(); i++) {
            if (s[i] == '(') {
                int startIndex = i + 1;
                int squareCount = 0;
                while (i < s.size()) {
                    if (s[i] == '(') squareCount++;
                    else if (s[i] == ')') squareCount--;

                    if (squareCount == 0) {
                        break;
                    }
                    i++;
                }

                curRes += sign * calculate(s.substr(startIndex, i - startIndex));
            }
            else if (isdigit(s[i])) {
                int startIndex = i;

                while (i < s.size() && isdigit(s[i + 1])) {
                    ++i;
                }

                curRes += sign * stoi(s.substr(startIndex, i - startIndex + 1));
            }
            else if (s[i] == '+') {
                sign = 1;
            }
            else if (s[i] == '-') {
                sign = -1;
            }
        }

        return curRes;
    }
};
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