<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:

        nums = []
        ops = []
        
        idx = 0
        while(idx<len(s)):
            if s[idx] in ('+', '-', '*', '/'):
                ops.append(s[idx])
                idx += 1
            elif s[idx] == ' ':
                idx += 1

            elif s[idx] in ('0', '1', '2', '3', '4', '5', '6','7', '8', '9'):
                cur_num = 0
                while (idx<len(s)) and (s[idx] in ('0', '1', '2', '3', '4', '5', '6','7', '8', '9')):
                    cur_num = cur_num * 10 + int(s[idx])
                    idx += 1
                nums.append(cur_num)

        stack = [nums[0]]
        if stack == ["0"]:
            return 0

        for i in range(len(ops)):
            if ops[i]  == '*':
                num1 = stack.pop()
                num2 = nums[i+1]
                res = num1 * num2
                stack.append(res)
            elif ops[i] == '/':
                num1 = stack.pop()
                num2 = nums[i+1]
                res = int(num1 / num2)
                stack.append(res)
            else:
                stack.append(nums[i+1])
        j = 0
        res = stack[0]
        for i in range(len(ops)):
            if ops[i] == '+':
                res += stack[j+1]
                j += 1
            elif ops[i] == '-':
                res-=stack[j+1]
                j += 1
            
        return int(res)
             
            
23000