<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>

import re

class Solution:
    def calculate(self, s: str) -> int:
        operands = list(map(int, re.split(r'[+\-*/]', s)))
        operators = [x.strip() for x in re.split(r'\d+', s) if x.strip() != ""]

        if not operators:
            return operands[0]

        print(operands)
        print(operators)

        operands_stack = []
        operators_stack = []
        calculated = None

        for i, o in enumerate(operators):
            if o in ["+", "-"]:
                if calculated != None:
                    print(calculated)
                    operands_stack.append(calculated)
                    calculated = None
                else:
                    operands_stack.append(operands[i])
                operators_stack.append(o)
            else:
                if calculated:
                    if o == "*":
                        calculated *= operands[i + 1]
                    else:
                        calculated = int(calculated / operands[i + 1])
                else:
                    if o == "*":
                        calculated = operands[i] * operands[i + 1]
                    else:
                        calculated = int(operands[i] / operands[i + 1])
        
        if calculated != None:
            operands_stack.append(calculated)
        else:
            operands_stack.append(operands[-1])

        print(operands_stack)
        print(operators_stack)

        if len(operands_stack) == 1:
            return operands_stack[0]

        res = None

        for i, o in enumerate(operators_stack):
            if res != None:
                if o == "+":
                    res += operands_stack[i + 1]
                else:
                    res -= operands_stack[i + 1]
            else:
                if o == "+":
                    res = operands_stack[i] + operands_stack[i + 1]
                else:
                    res = operands_stack[i] - operands_stack[i + 1]

        return res
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