<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 + 5
        # 3+2/5
        nums = []
        ops = []
        i = 0 
        n = len(s)
        while i < n:
            if s[i].isdigit():
                num = ""
                while i < n and s[i].isdigit():
                    num += s[i]
                    i += 1
                nums.append(int(num))
                i -= 1

                if ops and ops[-1] in "*/": 
                    num2 = nums.pop()
                    num1 = nums.pop()
                    op = ops.pop()
                    if op == "*":
                        nums.append(num1 * num2)
                    else: 
                        nums.append(int(num1 / num2))
            elif s[i] in "+-*/":
                ops.append(s[i])            
            i += 1

        # print(nums)
        # print(ops)

        nums=nums[::-1]
        for op in ops: 
            num1 = nums.pop()
            num2 = nums.pop()
            if op == "+":
                nums.append(num1 + num2)
            else: 
                nums.append(num1 - num2)
        
        return nums[0]
                


                

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