<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>

class Solution:
    def calculate(self, s: str) -> int:
        def priority(op):
            if op in ['+', '-']:
                return 1
            elif op in ['*', '/']:
                return 2
            else:
                return 0
        
        def toPostfix(infix):
            stack = ['']
            postfix = []
            i = 0
            prev_token = ''
            while i<len(infix):
                c = infix[i]
                if c.isdigit():
                    num = ''
                    while i<len(infix) and infix[i].isdigit():
                        num+=infix[i]
                        i+=1
                    postfix.append(num)
                    prev_token = 'number'
                    continue

                elif c == '(':
                    stack.append(c)
                    prev_token = '('
                elif c in "+-*/":
                    if c == '-' and (prev_token == '' or prev_token == "(" ):
                        postfix.append('0')  # 为一元负号添加一个0
                    else:
                        while priority(stack[-1]) >= priority(c):
                            postfix.append(stack.pop())
                    stack.append(c)
                    prev_token = 'operator'
                elif c == ')':
                    while stack[-1] != '(' and stack[-1] != '':
                        postfix.append(stack.pop())
                    stack.pop()
                    prev_token = ')'
                    
                i+=1
            while stack[-1] != '':
                postfix.append(stack.pop())
            print(postfix)
            return postfix
        
        def calPostfix(s):
            buffer = []
            for c in s:
                if c.isdigit():
                    buffer.append(int(c))
                elif c=='+':
                    res = buffer.pop()+buffer.pop()
                    buffer.append(res)
                elif c=='*':
                    res = buffer.pop()*buffer.pop()
                    buffer.append(res)
                elif c=='-':
                    num1 = buffer.pop()
                    num2 = buffer.pop()
                    buffer.append(num2-num1)
                elif c=='/':
                    num1 = buffer.pop()
                    num2 = buffer.pop()
                    buffer.append(num2//num1)
            return buffer[0]
        
        return calPostfix(toPostfix(s.strip()))
21486