<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 = []
        building = ''

        for character in s:
            if character.isnumeric():
                building += character
            else:
                if character in {'+', '-', '*', '/'}:
                    if building:
                        operations.append(building)
                        building = ''
                    operations.append(character)
        
        if building:
            operations.append(building)
            building = ''

        stack = []
        index = 0
        while index < len(operations):

            if operations[index] == '*':
                stack.append(int(stack.pop()) * int(operations[index + 1]))
                index += 1
            elif operations[index] == '/':
                stack.append(int(int(stack.pop()) / int(operations[index + 1])))
                index += 1
            elif operations[index] == '-':
                stack.append(int(operations[index + 1]) * -1)
                index += 1
            elif operations[index] != '+':
                stack.append(int(operations[index]))

            index += 1
        
        return sum(stack)


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