<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:
        s += '+'
        vals, ops = [], []
        curr_val = 0
        for char in s:
            if char in '0123456789':
                curr_val = (10 * curr_val) + int(char)
            if char in '+-*/':
                vals.append(curr_val)
                curr_val = 0
                ops.append(char)
        ops = ops[:-1]
        i = 0
        while i < len(ops):
            if ops[i] == '*':
                new_val = vals[i] * vals[i + 1]
                vals = vals[:i] + [new_val] + vals[i+2:]
                ops.remove('*')
            elif ops[i] == '/':
                new_val = vals[i] // vals[i + 1]
                vals = vals[:i] + [new_val] + vals[i+2:]
                ops.remove('/')
            else:
                i += 1
        
        res = vals[0]
        for op, val in zip(ops, vals[1:]):
            if op == '+': res += val
            else: res -= val

        return res
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