<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:
        res = 0
        def add(x):
            nonlocal res
            res += x
        def sub(x):
            nonlocal res
            res -= x

        i = 0
        action = add
        parsing_int = False
        int_state = 0
        while i < len(s):
            ch = s[i]

            if parsing_int and not (ord('0') <= ord(ch) <= ord('9')):
                action(int_state)
                parsing_int = False
                int_state = 0

            if ord('0') <= ord(ch) <= ord('9'):
                parsing_int = True
                int_state *= 10
                int_state += int(ch)
            elif s[i] == "+":
                action = add
            elif s[i] == "-":
                action = sub
            elif s[i] == "(":
                pairs = 1
                i += 1
                begin = i
                while pairs > 0:
                    if s[i] == "(":
                        pairs += 1
                    if s[i] == ")":
                        pairs -= 1
                    if pairs > 0:
                        i += 1
                action(self.calculate(s[begin:i]))
            i += 1
        if parsing_int:
            action(int_state)
        return res
            
51666