<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:
        
        # []
        # 3 + 2 * 2 -> 3 + 4 -> 7

        # 2 * 2 + 3
        # 
        # [3, +] -> 2*2 = 4, [3 + 4] -> [7]

        # 3 * 2 - 5 / 4
        # [6, -1] = 5
        s = s.replace(' ', '')
        s = re.split(r'(\+|\-|\*|/)', s)
        # print(s)

        stack = deque()
        idx = 0
        while idx < len(s):
            # print("stack:",stack)
            if s[idx] == "-":
                stack.append(int(s[idx+1]) * -1)
                idx += 1
            elif s[idx] == "+":
                stack.append(int(s[idx+1]))
                idx += 1
            elif s[idx] == "*":
                prev = stack.pop()
                after = int(s[idx+1])
                prod = prev * after
                stack.append(prod)
                idx += 1
            elif s[idx] == "/":
                prev = stack.pop()
                after = int(s[idx+1])
                prod = int(prev / after)

                # print("prod:",prod)
                stack.append(prod)
                idx += 1
            else:

                stack.append(int(s[idx]))
            idx += 1
        output = 0
        if stack:
            for i in stack:
                output += i
        return output


                    
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