<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:
        i = 0
        s = "".join([c for c in s if c != " "])
        l = len(s)
        stack = []
        res = 0
        while i < l:
            if stack and (stack[-1] == "/" or stack[-1] == "*"):
                ope = stack.pop()
                tmp = ""
                while i < l and s[i].isdigit():
                    tmp = tmp + s[i]
                    i += 1
                num2 = int(tmp)
                tmp = ""
                while stack and stack[-1].isdigit():
                    tmp = stack.pop() + tmp
                num1 = int(tmp)
                tmp_res = num1*num2 if ope == "*" else num1//num2
                for c in str(tmp_res): #[::-1]:
                    stack.append(c)
                # res += tmp_res
            if i < l:
                stack.append(s[i])
                i += 1
        res = 0
        i = 0
        l2 = len(stack)
        tmp = ""
        while i < l2 and stack[i].isdigit():  
            tmp += stack[i]
            i += 1
        if tmp:
            res = int(tmp)

        while i < l2:
            ope = stack[i]
            i += 1

            tmp = ""
            while i < l2 and stack[i].isdigit():  
                tmp += stack[i]
                i += 1
            
            num2 = int(tmp)
            res = res + num2 if ope == "+" else res - num2
        return res
            

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