<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:
        i, n = 0, len(s)
        ans, op = 0, '+'
        stack = deque()
        # (1+(4+5+2)-3)+(6+8)
        while i < n:
            if s[i] == ')':
                operand = 0
                exp = 0
                element = ')'
                while stack:
                    element = stack.pop()
                    if element == '(':
                        break
                    if element == '+':
                        exp += operand
                        operand = '#'
                    elif element == '-':
                        exp -= operand
                        operand = '#'
                    else:
                        operand = element
                if operand != '#':
                    exp += operand
                stack.append(exp)
            if s[i] in '0123456789': # 6
                num = int(s[i]) # 6
                i += 1
                while  i < n and s[i] in '0123456789':
                    num *= 10
                    num += int(s[i])
                    i += 1
                i -= 1
                stack.append(num)
            if i < n and  s[i] in '+-(':
                stack.append(s[i])
            i += 1 # 8
        
        operand = 0
        while stack:
            element = stack.pop()
            if element == '+':
                ans += operand
                operand = '#'
            elif element == '-':
                ans -= operand
                operand = '#'
            elif element == ' ':
                pass
            else:
                operand = element
        if operand != '#':
            ans += operand
        return ans
18343