<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:
        '''
        considerations
        - order of operations when evaluating
        - handling spaces, either remove or evaluate in-place
        - integer multiple digits

        implementation
        - use stack, evaluate all operations that are not addition, then return sum(stack) 
        - for each char
            - continue if space (no bearing on algorithm)
            - if num, add to current running int variable (can be multiple digits)
        - for each char that's an operation
            - operation only evaluated when another is found
            - if +-, add running int variable to stack as positive (+) or negative (-)
            - if */, evaluate { stack.pop() operation num }
        '''

        num = 0
        pre = '+'
        stack = []
        for char in s + '+':
            if char == ' ':
                continue
            elif char.isdigit():
                num = num * 10 + int(char)
            else:
                if pre == '+':
                    stack.append(num)
                elif pre == '-':
                    stack.append(-num)
                elif pre == '*':
                    stack.append(stack.pop() * num)
                elif pre == '/':
                    stack.append(math.trunc(stack.pop() / num))
                # expression's been evaluated so reset variables
                num = 0
                pre = char

        return sum(stack)
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