<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>

parenthesis = {"(", ")"}
operators = {"+", "-"}
non_digits = parenthesis.union(operators)


class Solution:
    def calculate(self, s: str) -> int:
        s_without_spaces = self.remove_spaces(s)
        stack = []

        for char in s_without_spaces:
            if char != ")":
                stack.append(char)
                continue

            result = 0

            while stack[-1] != "(":
                number = self.pop_number(stack)
                result = self.perform_operation(result, number, "+")

            stack.pop()

            for digit in str(result):
                stack.append(digit)

        return self.evaluate_simple_expr(stack)

    def evaluate_simple_expr(self, stack):
        result = 0

        while len(stack) > 0:
            number = self.pop_number(stack)
            result = self.perform_operation(result, number, "+")

        return result

    def pop_number(self, stack):
        reversed_number = ""

        while (len(stack) > 0) and (stack[-1] not in non_digits):
            reversed_number += stack.pop()

        number = int(self.reverse_str(reversed_number))

        while len(stack) > 0 and (stack[-1] in operators):
            operation = stack.pop()
            number = self.perform_operation(0, number, operation)

        return number

    def perform_operation(self, x, y, operation):
        if operation == "+":
            return x + y

        return x - y

    def reverse_str(self, s):
        n = len(s)
        reversed_str = ""

        for i in range(n):
            reversed_str += s[n - 1 - i]

        return reversed_str

    def remove_spaces(self, s):
        s_without_spaces = ""

        for char in s:
            if char != " ":
                s_without_spaces += char

        return s_without_spaces
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