<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:
        """
        s = "3-2*2"
        prev_num = -4
        curr_num = 0
        res = 3
        prev_operation = *

        s = "3/2"
        prev_num = 1.5
        curr_num = 0
        res = 1.5
        prev_operation = 2

        s = "1+1+1"
        prev_num = 1
        curr_num = 0
        res = 2
        prev_operation = +

        1. Perfrom the previous operation when we've encountered the current one (to ensure precedence rules are met).
        2. Four vars: prev_num, curr_num, res (hold the +/- computed answers), prev_operation.
        3. Iterate through s:
            - When char is number: update the curr_num = curr_num*10 + int(s[i]).
            - When the char is an operation (not space) OR we arrived at end of s:
                - If the operation is +/-, store the prev_num in res, update the prev_num to = curr_num (positive when prev_operation is +, else negative).
                - If operation is *, prev_num = prev_num * curr_num.
                - If operation is /, prev_num = prev_num / curr_num.
                - curr_num is reset, update the prev_operation.
        """
        prev_num, curr_num, ret, prev_operation = 0, 0, 0, "+"
        for i in range(len(s)):
            char = s[i]
            if char.isnumeric():
                curr_num = curr_num*10 + int(char)
            if (not char.isnumeric() and char != " ") or i == len(s) - 1:
                if prev_operation in ["+", "-"]:
                    ret += prev_num
                    prev_num = curr_num if prev_operation == "+" else -curr_num
                elif prev_operation == "*":
                    prev_num *= curr_num
                elif prev_operation == "/":
                    tmp_quotient = prev_num / curr_num
                    if tmp_quotient < 0:
                        prev_num = math.ceil(tmp_quotient)
                    else:
                        prev_num = math.floor(tmp_quotient)
                curr_num = 0
                prev_operation = char

        print(ret, prev_num)
        ret += prev_num
        return ret
18800