<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:
        operations = ["+", "-", "*", "/"]
        operation_order = []
        for c in s:
            if c in operations:
                operation_order.append(c)
        
        for o in operations:
            s = ",".join(s.split(o))

        str_nums = s.split(",")

        nums = [int(n.strip()) for n in str_nums ]

        for i in range(len(operation_order)):
            if operation_order[i] == "*":
                nums[i + 1] = nums[i] * nums[i + 1]
                nums[i] = None
                operation_order[i] = "d"
            elif operation_order[i] == "/":
                nums[i + 1] = nums[i] // nums[i + 1]
                nums[i] = None
                operation_order[i] = "d"

        for i in range(len(operation_order)):
            if operation_order[i] == "+":
                curr = i + 1
                while nums[curr] == None:
                    curr += 1
                nums[curr] = nums[i] + nums[curr]
            elif operation_order[i] == "-":
                curr = i + 1
                while nums[curr] == None:
                    curr += 1
                nums[curr] = nums[i] - nums[curr]

        return nums[-1]    
24900