<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 = []       # The robot uses a stack to keep track of numbers and operators
        num = 0          # This is where the robot reads numbers
        op = '+'         # The robot starts with addition (+)
        i = 0
        
        # The robot starts reading the math problem
        while i < len(s):
            ch = s[i]     # The robot looks at each character one by one

            if ch.isdigit():  # If the robot finds a number, it reads the whole number
                num = 0
                while i < len(s) and s[i].isdigit():
                    num = num * 10 + int(s[i])  # The robot builds the number
                    i += 1
                i -= 1  # Adjust to move back after reading the number

                if op == '+' or op == '-':  # If the robot has + or -, it remembers the number
                    stack.append(num if op == '+' else -num)
                elif op == '*':  # If it’s *, the robot multiplies
                    stack.append(stack.pop() * num)
                elif op == '/':  # If it’s /, the robot divides
                    stack.append(int(stack.pop() / num))

            elif ch in '+-*/':  # If the robot sees an operator, it remembers it for later
                op = ch
            i += 1
        return sum(stack)

        
20000