<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>

import math
class Solution:
    def calculate(self, ss: str) -> int:
        current_number = ''
        ops = None
        num1 = None
        cache = []
        for s in ss:
            if s == " ":
                continue
            if s.isnumeric():
                current_number+=s
            else:
                
                cache.append(int(current_number))
                current_number = ''

                if ops is not None and num1 is not None:
                    num2 = cache.pop(-1)
                    if ops == '*':
                        cache.append(num1*num2)
                    else:
                        cache.append(math.floor(num1/num2))
                    num1=ops=None

                if s in ['+','-']:   
                    cache.append(s)
                else: 
                    ops  = s
                    num1 = cache.pop(-1)

        cache.append(int(current_number))
          
        if ops is not None and num1 is not None:

            num2 = cache.pop(-1)
            
            if ops == '*':
                cache.append(num1*num2)
            else:
                cache.append(math.floor(num1/num2))
        
        out = cache[0]
        sign = None
        for item in cache[1:]:
            if item in ['+','-']:
                sign = item
            else:
                if sign=='+':
                    out += item
                elif sign == '-':
                    out -= item
        return out
    
23700