<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:
        stack=[]
        operation = None
        cant_add = False
        for item in s.replace(" ",""):
            if item in "+-*/":
                if operation is not None:
                    num2 = int(stack.pop())
                    num1 = int(stack.pop())
                    if operation=="*":
                        stack.append(str(num1*num2))
                    else:
                        stack.append(str(num1//num2))
                    operation=None
                if item in "+-":
                    stack.append(item)
                else:
                    operation=item
                    cant_add=True
            else:
                if stack and stack[-1] not in "+-*/" and not(cant_add):
                    stack[-1]+=item
                else:
                    stack.append(item)
                cant_add=False
        print(stack)
        if operation is not None:
            num2 = int(stack.pop())
            num1 = int(stack.pop())
            if operation=="*":
                stack.append(str(num1*num2))
            else:
                stack.append(str(num1//num2))

        operation="+"
        ans=0
        for item in stack:
            if item not in "+-":
                if operation=="+":
                    ans+=int(item)
                else:
                    ans-=int(item)
            else:
                operation=item
        return ans


            

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