<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 = s.replace(" ","")

        def operate(int1: int, int2: int, ope: str):
            if ope == "*":
                return int(int1*int2)
            if ope == "/":
                return int(int1/int2)
            if ope == "+":
                return int1+int2
            if ope == "-":
                return int1-int2

        p1, p2, highLvOperators, lowLvOperators, ope, num = 0, 0, ["*","/"], ["+","-"], [], []
        while p2<len(s):
            if s[p2] in highLvOperators or s[p2] in lowLvOperators:
                num.append(int(s[p1:p2]))
                ope.append(s[p2])
                p1 = p2+1
            p2 += 1
        num.append(int(s[p1:p2]))
        
        res1, res2, tempOpe, isLastHighLv = 0, num[0], "+", False
        for i in range(0, len(ope)):
            if ope[i] in highLvOperators:
                res2 = operate(res2,num[i+1],ope[i])
            else:
                res1 = operate(res1,res2,tempOpe)
                res2 = num[i+1]
                tempOpe = ope[i]
        return int(operate(res1,res2,tempOpe))
            


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