<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:
        final_stack = []
        sign_stack = []
        temp_num = []
        s = "".join(s.split())
        for i in range(len(s)):
            # print(final_stack, sign_stack, s[i])
            if s[i] in ["0", "1", "2", "3", "4", "5", "6", "7", "8", "9"]:
                temp_num.append(s[i])
            else:
                if temp_num:
                    final_stack.append(int("".join(temp_num)))
                    temp_num = []
            if s[i] == "-":
                # print("L20 {}, {}".format(s[i], s[i-1]))
                if i==0 or s[i-1]=="(":
                    sign_stack.append("^")
                else:
                    while sign_stack and sign_stack[-1] in ["+", "-", "*", "/", "^"]:
                        final_stack.append(sign_stack.pop())
                    sign_stack.append("-")
            elif s[i] == "+":
                while sign_stack and sign_stack[-1] in ["+", "-", "*", "/", "^"]:
                    final_stack.append(sign_stack.pop())
                sign_stack.append("+")
            elif s[i] in ["*", "/"]:
                while sign_stack and sign_stack[-1] in ["*", "/", "^"]:
                    final_stack.append(sign_stack.pop())
                sign_stack.append(s[i])
            if s[i] == "(":
                sign_stack.append("(")
            elif s[i] == ")":
                while sign_stack[-1] != "(":
                    final_stack.append(sign_stack.pop())
                sign_stack.pop()
        if temp_num:
            final_stack.append(int("".join(temp_num)))
            temp_num = []
        while sign_stack:
            final_stack.append(sign_stack.pop())
        # print(final_stack)
        result_stack = []
        for x in final_stack:
            if x == "+":
                b, a = result_stack.pop(), result_stack.pop()
                result_stack.append(a+b)
            elif x == "-":
                b, a = result_stack.pop(), result_stack.pop()
                result_stack.append(a-b)
            elif x == "^":
                a = result_stack.pop()
                result_stack.append(-a)
            elif x == "*":
                b, a = result_stack.pop(), result_stack.pop()
                result_stack.append(a*b)
            elif x == "/":
                b, a = result_stack.pop(), result_stack.pop()
                result_stack.append(int(a/b))
            else:
                result_stack.append(x)
        return int(result_stack[0])
23100