<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 Calc:
    def __init__(self):
        self.x = 0
        self.buf = []
    
    def op(self, sign, y):
        self.buf.append((sign, y))
        return self

    def eval(self):
        for el in self.buf:
            res = el[1] if type(el[1]) == int else el[1].eval().x
            if el[0] == "+":
                self.x += res
            elif el[0] == "-":
                self.x -= res

        self.buf = []
        return self

class Solution:
    def calculate(self, s: str) -> int:
        pre_calcs = []
        calc = Calc()

        op = "+"
        i = -1
        l = len(s)

        while True:
            i += 1
            if i >= l:
                break
                
            num_buf = []
            while s[i] in "0123456789":
                num_buf.append(s[i])
                i += 1
                if i == l:
                    break
            if num_buf:
                num = int("".join(num_buf))
                calc.op(op, num)
            
            if i == l:
                break
            
            ch = s[i]
            if ch == "(":
                pre_calc = calc
                pre_calcs.append(pre_calc)
                calc = Calc()
                pre_calc.op(op, calc)
                op = "+"
            elif ch == ")":
                calc = pre_calcs.pop()
            elif ch == " ":
                continue
            elif ch == "+":
                op = "+"
            elif ch == "-":
                op = "-"

        calc.eval()
        return calc.x        
        
        # 3+3
        # -(6+6)
        # 3+(2-3)

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