<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:
        s = '(' + s + ')'
        p_stk = [0]
        stk = []
        num = 0
        num_used = False
        for c in s:
            if c.isdigit():
                num *= 10
                num += int(c)
                num_used = True
            else:
                if num_used:
                    stk.append(num)
                    num = 0
                    num_used = False
                if c == '(':
                    p_stk.append(len(stk))
                    stk.append('(')
                elif c == '+':
                    stk.append(c)
                elif c == '-':
                    if not stk or type(stk[-1]) != int:
                        stk.append(0)
                    stk.append(c)
                elif c == ')':
                    st = p_stk[-1] + 1
                    comp = stk[st]
                    for i in range(st + 1,len(stk),2):
                        if stk[i] == '+':
                            comp += stk[i+1]
                        else:
                            comp -= stk[i+1]
                    stk = stk[:st-1]
                    stk.append(comp)
                    p_stk.pop()
        return stk[0]
            

                

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