<p>Given a string <code>s</code> representing a valid expression, implement a basic calculator to evaluate it, and return <em>the result of the evaluation</em>.</p>

<p><strong>Note:</strong> You are <strong>not</strong> 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 = &quot;1 + 1&quot;
<strong>Output:</strong> 2
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> s = &quot; 2-1 + 2 &quot;
<strong>Output:</strong> 3
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;(1+(4+5+2)-3)+(6+8)&quot;
<strong>Output:</strong> 23
</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 digits, <code>&#39;+&#39;</code>, <code>&#39;-&#39;</code>, <code>&#39;(&#39;</code>, <code>&#39;)&#39;</code>, and <code>&#39; &#39;</code>.</li>
	<li><code>s</code> represents a valid expression.</li>
	<li><code>&#39;+&#39;</code> is <strong>not</strong> used as a unary operation (i.e., <code>&quot;+1&quot;</code> and <code>&quot;+(2 + 3)&quot;</code> is invalid).</li>
	<li><code>&#39;-&#39;</code> could be used as a unary operation (i.e., <code>&quot;-1&quot;</code> and <code>&quot;-(2 + 3)&quot;</code> is valid).</li>
	<li>There will be no two consecutive operators in the input.</li>
	<li>Every number and running calculation will fit in a signed 32-bit integer.</li>
</ul>

import re

op = ['+', '-', '*', '/']
def isNumber(s : str) -> bool:
    # Regular expression to match integer
    pattern = r'^-?\d+$'
    return bool(re.match(pattern, s))

class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        res = []
        for tok in tokens:
            if tok not in op:
                res.append(int(tok))
            else:
                v2 = int(res.pop())
                v1 = int(res.pop())
                next = 0
                if (tok == '+'):
                    next = v1 + v2
                if (tok == '-'):
                    next = v1 -v2
                if (tok == '*'):
                    next = v1 * v2
                if (tok == '/'):
                    next = v1 / v2
                res.append(int(next))
        
        return res[0]

    def convertToRPN(self, exp: str) -> List[str]:
        tokens = [char for char in exp if char != ' ']
        # print(tokens)
        # now we need to merge the digits into whole numbers again
        cur_num = ""
        whole = []
        for char in tokens:
            if char.isdigit():
                cur_num += char
            else:
                if (cur_num != ""):
                    whole.append(cur_num)
                whole.append(char)
                cur_num = ""
        
        if cur_num != "":
            whole.append(cur_num)

        # Handling unary minus
        final = []
        prev = "+"
        last = False
        for i in range(len(whole)):
            token = whole[i]
            if (prev == "+" or prev == "-" or prev == "(") and token == "-":
                # Treat this "-" as a unary operator
                final.append("0")
            final.append(token)
            prev = token
        print(whole)
        print(final)
        # simplified shunting-yard algorithm (without precedence)
        res = []
        ops = []
        for token in final:
            if token == '+' or token == '-':
                while ops and ops[-1] != '(':
                    res.append(ops.pop())
                ops.append(token)
            elif token == '(':
                ops.append(token)
            elif token == ')':
                while ops[-1] != '(':
                    res.append(ops.pop())
                ops.pop() # '('
            else:
                res.append(token) # number

        while ops:
            res.append(ops.pop())
        
        return res

    def calculate(self, s: str) -> int:
        print(self.convertToRPN(s))
        return self.evalRPN(self.convertToRPN(s))
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