<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>

OPERATORS = {
        "+": lambda x, y: x + y,
        "-": lambda x, y: x - y,
        "*": lambda x, y: x * y,
        "/": lambda x, y: int(x / y)
    }

DIGITS = {c for c in '1234567890'}

class Solution:    

    def parse(self, expr: str):
        parsed_expression = []
        number_accumulator = []
        for c in expr:
            if c in OPERATORS:
                if number_accumulator:
                    parsed_expression.append(int(''.join(number_accumulator)))
                    number_accumulator = []
                parsed_expression.append(c)
            elif c in DIGITS:
                number_accumulator.append(c)
            else:
                # whitespace
                pass
        if number_accumulator:
            parsed_expression.append(int(''.join(number_accumulator)))
        
        return parsed_expression


    def calculate(self, s: str) -> int:
        expression = self.parse(s)
        
        currentNumber = 0
        operation = "+"
        stack = []

        for element in expression:
            if element in OPERATORS:
                operation = element
            else:
                # Number
                if operation == "-":
                    stack.append(- element)
                elif operation == "+":
                    stack.append(element)
                else:
                    # We can do the multiplication or division now
                    left_operand = stack.pop()
                    stack.append(OPERATORS[operation](left_operand, element))
        return sum(stack)

22600