<p>Given a string <code>s</code> which represents an expression, <em>evaluate this expression and return its value</em>.&nbsp;</p>

<p>The integer division should truncate toward zero.</p>

<p>You may assume that the given expression is always valid. All intermediate results will be in the range of <code>[-2<sup>31</sup>, 2<sup>31</sup> - 1]</code>.</p>

<p><strong>Note:</strong> You are not 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 = "3+2*2"
<strong>Output:</strong> 7
</pre><p><strong class="example">Example 2:</strong></p>
<pre><strong>Input:</strong> s = " 3/2 "
<strong>Output:</strong> 1
</pre><p><strong class="example">Example 3:</strong></p>
<pre><strong>Input:</strong> s = " 3+5 / 2 "
<strong>Output:</strong> 5
</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 integers and operators <code>(&#39;+&#39;, &#39;-&#39;, &#39;*&#39;, &#39;/&#39;)</code> separated by some number of spaces.</li>
	<li><code>s</code> represents <strong>a valid expression</strong>.</li>
	<li>All the integers in the expression are non-negative integers in the range <code>[0, 2<sup>31</sup> - 1]</code>.</li>
	<li>The answer is <strong>guaranteed</strong> to fit in a <strong>32-bit integer</strong>.</li>
</ul>

class Solution:
    def calculate(self, s: str) -> int:
        if "+" in s:
            res = None
            for group in s.split("+"):
                group = group.strip()
                if res is None:
                    res = self.calculate(group)
                else:
                    res += self.calculate(group)
            return res
        if "-" in s:
            res = None
            for group in s.split("-"):
                group = group.strip()
                if res is None:
                    res = self.calculate(group)
                else:
                    res -= self.calculate(group)
            return res
        
        res = None
        operation = None
        digits = []
        for char in s:
            match char:
                case "*" | "/":
                    number = int("".join(digits))
                    digits = []
                    if res is None:
                        res = number
                    else:
                        assert operation is not None
                        if operation == "*":
                            res *= number
                        else:
                            res = int(res / number)
                    operation = char
                case _:
                    digits.append(char)
        if len(digits) > 0:
            number = int("".join(digits))
            if operation is None:
                return number
            if operation == "*":
                res *= number
            else:
                res = int(res / number)
        return res

23000