<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 perform_op(self, x, y, op) -> int:
        if op == '/':
            ans = int(x)//int(y)
        elif op == "*":
            ans = int(x) * int(y)
        elif op == "+":
            ans = int(x) + int(y)
        else:
            ans = int(x) - int(y)

        return str(ans)

    def calculate(self, s: str) -> int:
        s = s.replace(' ', '')
        digits = [str(i) for i in range(0, 10)]

        values = []
        ops = []

        i = 0
        len_s = len(s)
        while i < len_s:
            if s[i] in digits:
                num = ""
                while i < len_s and s[i] in digits:
                    num += s[i]
                    i += 1
                values.append(num)
                continue
            else:
                if s[i] in ['*', '/']:
                    while ops and ops[-1] in ['*', '/']:
                        op = ops.pop()
                        second = values.pop()
                        first = values.pop()
                        ans = self.perform_op(first, second, op)
                        values.append(ans)
                elif s[i] in ['+', '-']:
                    while ops and ops[-1] in ['*', '/', '+', '-']:
                        op = ops.pop()
                        second = values.pop()
                        first = values.pop()
                        ans = self.perform_op(first, second, op)
                        values.append(ans)
        

                ops.append(s[i])
                i += 1


        while ops:
            op = ops.pop()
            second = values.pop()
            first = values.pop()
            ans = self.perform_op(first, second, op)
            values.append(ans)

        return int(values[0])



18900