<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 = s.replace(' ', '')

        ops = []
        while s:
            foundSep = False
            for i, c in enumerate(s):
                if c in {'+', '-', '*', '/'}:
                    ops.extend([int(s[:i]), s[i]])
                    foundSep = True
                    s = s[i + 1:]
                    break

            if not foundSep:
                ops.append(int(s))
                s = None

        for i, c in enumerate(ops):
            if c == '*':
                ops[i + 1] = ops[i - 1] * ops[i + 1]
                ops[i - 1] = ops[i] = None
            elif c == '/':
                ops[i + 1] = ops[i - 1] // ops[i + 1]
                ops[i - 1] = ops[i] = None

        rv = 0
        next_op = None
        for op in ops:
            if not op:
                continue
            elif isinstance(op, int):
                if next_op is None:
                    rv = op
                elif next_op == '+':
                    rv += op
                elif next_op == '-':
                    rv -= op
            else:
                next_op = op
        
        return rv



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