<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>

import re
class Expression:
    def __init__(self): 
        self.sub_expressions = []
        self.operators = []

    def evaluate(self):
        if len(self.operators) == 0:
            if isinstance(self.sub_expressions[0], int):
                return self.sub_expressions[0]
            else:
                return self.sub_expressions[0].evaluate()
        else:
            if len(self.sub_expressions) == len(self.operators):
                self.sub_expressions.insert(0, Operand(0))
            a0 = self.sub_expressions[0].evaluate()
            for i in range(len(self.operators)):
                if self.operators[i] == '+':
                    a0 += self.sub_expressions[i+1].evaluate()
                else:
                    a0 -= self.sub_expressions[i+1].evaluate()
            return a0
    def add_operand(self, operand):
        self.sub_expressions.append(operand)
    
    def add_operator(self, operator):
        self.operators.append(operator)

class Operand(Expression):
    def __init__(self, number):
        self.value  = number
    def evaluate(self):
        return self.value

class Solution:
    def calculate(self, s: str) -> int:
        pattern = r'(\d+|\+|\-|\(|\))'
        parts = re.findall(pattern, s)

        stack =  []
        stack.append(Expression())
        current_expression = stack[0]
        for part in parts :
            if part == ' ':
                continue
            elif part == '(':
                stack.append(Expression())
                current_expression = stack[-1]
            elif part == ')':
                temp = stack.pop()
                current_expression = stack[-1]
                current_expression.add_operand(temp)
            elif part == '+':
                current_expression.add_operator('+')
            elif part == '-':
                current_expression.add_operator('-')
            else:
                operand = int(part)
                current_expression.add_operand(Operand(operand))

        return stack[0].evaluate()
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