<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(' ', '')
        i = 0
        toks = []
        while i < len(s):
            if s[i] in '+-*/':
                toks.append(s[i])
                i += 1
            else:
                j = i
                while j < len(s) and s[j].isdigit():
                    j += 1
                num = int(s[i:j])
                i = j
                if toks:
                    if toks[-1] == '*':
                        toks.pop()
                        toks.append(toks.pop() * num)
                    elif toks[-1] == '/':
                        toks.pop()
                        toks.append(toks.pop() // num)
                    else:
                        toks.append(num)
                else:
                    toks.append(num)
        # print(toks)
        if len(toks) == 1:
            return toks[0]
        total = 0
        for i in range(1, len(toks)-1, 2):
            if toks[i] == '+':
                total = toks[i+1] = toks[i-1] + toks[i + 1]
            elif toks[i] == '-':
                total = toks[i+1] = toks[i-1] - toks[i+1]
        return total

23900