<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 re
class Solution:
    def calculate(self, s: str) -> int:
        number = "[0-9]"
        exp = []
        last_index = len(s)-1
        pointer = 0
        bad_char = [" ", '', ' ']
        for i in range(0,len(s)):
            if re.match(number ,s[i]):
                if i == last_index:
                    exp.append(s[pointer:])
            elif s[i] in bad_char and i == last_index:
                exp.append(s[pointer:])
                pointer = i+1
            else:
                if s[i] not in bad_char:
                    exp.append(s[pointer:i])
                    pointer = i+1
                    exp.append(s[i])
        if len(exp) == 1 and exp[0] not in ['*', '/', '+', '-']:
            return int(exp[0])
        stack = []
        for index, i in enumerate(exp):
            if i == '+':
                stack.append(int(exp[index-1]))
            elif i == '-':
                stack.append(int(exp[index-1]))
                exp[index+1] = int(exp[index+1])*(-1)
            elif i == '*':
                exp[index+1] = int(exp[index-1]) * int(exp[index+1])
            elif i == '/':
                exp[index+1] = int(exp[index-1]) / int(exp[index+1])
        stack.append(int(exp[-1]))
        output = 0
        for j in stack:
            output += j


        return math.floor(output)
24400