<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 Node:
    def __init__(self, op, left=None, right=None):
        self.op = op
        self.left = left
        self.right = right

class Solution:
    def exprTree(self, chars):
        stack = []
        root = None
        idx = 0
        N = len(chars)
        while idx < N:
            ch = chars[idx]
            if ch == ' ':
                idx += 1
                continue
            elif ch == '-':
                if not root: 
                    root = Node('-', 0)
                else:
                    root = Node('-', root)
                idx += 1
            elif ch == '+':
                root = Node('+', root)
                idx += 1
            elif ch == '(':
                if root:
                    stack.append(root)
                root = None
                idx += 1
            elif ch == ')':
                if stack:
                    node = stack.pop()
                    node.right = root
                    root = node
                idx += 1
            else:
                buf = ''
                while idx < N and chars[idx].isdigit():
                    buf += chars[idx]
                    idx += 1
                buf = int(buf)
                if not root:
                    root = Node('+', 0, buf)
                else:
                    root.right = buf
        return root
    def evaluate(self, root):
        writeStack = []
        readStack = [root.op, root.left, root.right]
        while readStack:
            top = readStack.pop()
            if isinstance(top, int):
                writeStack.append(top)
            elif isinstance(top, str):
                a = writeStack.pop()
                b = writeStack.pop()
                if top == '+': opt = a+b
                else: opt = a-b
                writeStack.append(opt)
            else:
                readStack.append(top.op)
                readStack.append(top.left)
                readStack.append(top.right)
        return writeStack[0]

    def calculate(self, s: str) -> int:
        root = self.exprTree(s)
        opt = self.evaluate(root)
        return opt

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