<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>

"""

[1]

res = 0

"""

class Solution:
    def calculate(self, s: str) -> int:
        s = s.replace(" ", "")
        stack = deque()
        i = 0
        res = 0

        while i < len(s):
            if s[i] in ["+", "-"]:
                stack.append(s[i])
                i += 1
                continue
            if s[i] in ["*", "/"]:
                left = stack.pop()
                op = s[i]
                right = ""
                i += 1
                while i < len(s) and s[i].isdigit():
                    right += s[i]
                    i += 1
                right = int(right)
                if op == "*":
                    stack.append(left * right)
                else:
                    stack.append(int(left / right))
                continue
            if s[i].isdigit():
                d = ""
                while i < len(s) and s[i].isdigit():
                    d += s[i]
                    i += 1
                d = int(d)
                stack.append(d)
        print(stack)
        while len(stack) > 0:
            left = stack.popleft()
            if len(stack) == 0:
                res = left
                break
            op = stack.popleft()
            right = stack.popleft()
            if op == "+":
                stack.appendleft(left + right)
            elif op == "-":
                stack.appendleft(left - right)
            
        return res
        
24600