<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:
        left, right = "", ""
        operator = None
        i = 0
        while i < len(s):
            c = s[i]
            if c == " ":
                i += 1
                continue
            if c in ["+", "-", "*", "/"]:
                if operator is None:
                    operator = c
                    i += 1
                    continue
                if operator in ["*", "/"]:
                    left = self.operate(int(left), int(right), operator)
                    right = ""
                    operator = c
                else:
                    if c in ["+", "-"]:
                        left = self.operate(int(left), int(right), operator)
                        right = ""
                        operator = c
                    else:
                        rightt = ""
                        while i + 1 < len(s) and s[i + 1] not in ["+", "-", "*", "/"]:
                            if s[i + 1] != " ":
                                rightt += s[i + 1]
                            i += 1
                        right = self.operate(int(right), int(rightt), c)
                        print("!")
            else:
                if operator is None:
                    left += c
                else:
                    right += c
            i += 1
        if not right:
            return int(left)
        return int(self.operate(int(left), int(right), operator))

    def operate(self, l, r, op):
        if op == "+":
            return str(l + r)
        elif op == "-":
            return str(l - r)
        elif op == "*":
            return str(l * r)
        elif op == "/":
            return str(int(l / r))
18800