<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 Calculator:
    def __init__(self, tokens):
        self.tokens = tokens
        self.i = 0

    def evaluate(self):
        return self.evaluate_term()

    def evaluate_term(self):
        result = self.evaluate_unary()
        while self.i < len(self.tokens) and self.tokens[self.i] in "+-":
            operator = self.tokens[self.i]
            self.i += 1
            right_result = self.evaluate_unary()
            if operator == "+":
                result += right_result
            else:
                result -= right_result
        return result

    def evaluate_unary(self):
        if self.tokens[self.i] == "-":
            operator = self.tokens[self.i]
            self.i += 1
            return -self.evaluate_unary()
        return self.evaluate_primary()

    def evaluate_primary(self):
        if self.tokens[self.i] == "(":
            self.i += 1
            result = self.evaluate()
            self.i += 1
            return result
        result = int(self.tokens[self.i])
        self.i += 1
        return result

class Solution:
    def calculate(self, s: str) -> int:
        tokens = self.tokenize(s)
        return Calculator(tokens).evaluate()

    def tokenize(self, s):
        i = 0
        expected_tokens = "+-()"
        tokens = []
        while i < len(s):
            if s[i].isdigit():
                start = i
                while i < len(s) and s[i].isdigit():
                    i += 1
                tokens.append(s[start:i])
            elif s[i] in expected_tokens:
                tokens.append(s[i])
                i += 1
            else:
                i += 1

        return tokens


        
20858