<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:
    def calculate(self, s: str) -> int:
        sum  = 0
        index = 0
        subtract = False

        while index < len(s):
            if s[index] == "(":
                num_brackets = 1
                end = index
                while end < len(s) and num_brackets != 0:
                    end += 1
                    if s[end] == "(":
                        num_brackets += 1
                    elif s[end] == ")":
                        num_brackets -= 1
                sub_sum = self.calculate(s[index+1:end])
                if subtract and sub_sum < 0:
                    sum += (-sub_sum)
                elif subtract and sub_sum >= 0:
                    sum -= sub_sum
                else:
                    sum += sub_sum
                if end-1 != index:
                    index = end-1

            elif s[index] == "-":
                subtract = True

            elif s[index] == "+":
                subtract = False

            elif s[index].isdigit():
                end = index
                while end < len(s) and s[end].isdigit():
                    end += 1
                if subtract:
                    sum -= int(s[index:end])
                else:
                    sum += int(s[index:end])
                if end-1 != index:
                    index = end-1
            index += 1

        return sum

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