<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_nums = []
        stack_operations = []
        i = 0
        while i < len(s):
            if s[i].isdigit():
                j = i + 1
                while j < len(s) and s[j].isdigit():
                    j += 1
                num = int(s[i:j])
                i = j
                if len(stack_nums) == 0:
                    stack_nums.append(num)
                else:
                    operation = stack_operations[-1]
                    if operation == '/':
                        stack_nums[-1] = stack_nums[-1] // num
                        stack_operations.pop(-1)
                    elif operation == '*':
                        stack_nums[-1] = stack_nums[-1] * num
                        stack_operations.pop(-1)
                    else:
                        stack_nums.append(num)
            elif s[i] in {'+', '-', '*', '/'}:
                stack_operations.append(s[i])
                i += 1
            else:
                i += 1
        print(stack_nums)
        print(stack_operations)
        ans = stack_nums[0]
        for i, op in enumerate(stack_operations):
            if op == '+':
                ans += stack_nums[i+1]
            else:
                ans -= stack_nums[i+1]
        return ans

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