<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:
        operations = set(['+', '-', '*', '/'])

        # parse string into array of numbers and operators
        currentNum = ""
        arr = []
        for i in range(len(s)):
            if s[i].isdigit():
                currentNum += s[i]
            elif s[i] in operations:
                arr.append(currentNum)
                arr.append(s[i])
                currentNum = ""
        arr.append(currentNum)

        # iterate through the array and do multiplication and division operations
        nums, operators = [], []
        operation = ''

        for current in arr:
            if current.isdigit():
                current = int(current)
                if operation == '+' or operation == '-':
                    nums.append(current)
                    operators.append(operation)
                elif operation == '*':
                    nums[-1] *= current
                elif operation == '/':
                    nums[-1] //= current
                else:
                    nums.append(current)
                operation = ''
            else:
                operation = current

        # iterate through the array and do addition and subtraction operations
        total = nums[0]
        i, j = 1, 0
        while i < len(nums):
            if operators[j] == '+':
                total += nums[i]
            else:
                total -= nums[i]
            i, j = i + 1, j + 1
        return total
23500