<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:
    # n = iter(range(0, 10))
    def calculate(self, s: str) -> int:
        return self.recur(s.replace(' ', ''), 0)[1]

    def recur(self, s: str, index: int):
        # n = next(self.n)
        stack = []
        continuous_number = False
        negative_state = False
        while index < len(s):
            symbol = s[index]
            if symbol.isdigit():
                if continuous_number:
                    stack[-1] += symbol
                else:
                    stack.append(symbol)
                continuous_number = True
            else:
                if symbol == '-':
                    if s[index + 1].isdigit():
                        stack.append('-' + s[index + 1])
                        index += 1
                        continuous_number = True
                    else:
                        negative_state = not negative_state
                        continuous_number = False
                elif symbol == '(':
                    index, res = self.recur(s, index + 1)
                    stack.append(res * (-(2 * negative_state - 1)))
                    negative_state = False
                    continuous_number = False
                elif symbol == ')':
                    # print(sum(list(map(int, stack))))
                    return index, sum(list(map(int, stack)))
                else:
                    continuous_number = False
            index += 1
            # print(n, stack)

        return index, sum(map(int, stack))


# print(Solution().calculate("2-4-(8+2-6+(8+4-(1)+8-10))"))
# # print('\')
# print(Solution().calculate("(1+(4+5+2)-3)+(6+8)")[1])
# (exec("print((1+(4+5+2)-3)+(6+8))"))
# print(Solution().calculate("2-1 - 2"))[1]

18971