<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:
        n = len(s)
        stack = []
        i = 0
        while i < n: 
            if s[i] == ' ': 
                i += 1
            elif s[i].isdigit(): 
                ans = 0
                while i < n and s[i].isdigit(): 
                    ans = ans * 10 + int(s[i])
                    i += 1
                while stack and stack[-1] in ('*', '/'): 
                    operator = stack.pop()
                    operand = stack.pop()
                    if operator == '*': 
                        ans = operand * ans
                    else: 
                        ans = operand // ans
                stack.append(ans)
            else: 
                stack.append(s[i])
                i += 1
        temp = []
        while stack: 
            if stack[-1] not in ('+', '-'): 
                operand = stack.pop()
                if stack and stack[-1] == '-': 
                    stack.pop()
                    operand *= (-1)
                temp.append(operand)
            else: 
                stack.pop()
        print(temp)
        return sum(temp)


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