<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:
        stack = []
        i=0
        while(i<len(s)):
            if s[i] == ' ':
                pass
            elif s[i] in ['+','-','*','/']:
                temp = ""
                while(stack and stack[-1] not in ['+','-','*','/']):
                    temp =  stack.pop() + temp
                stack.append(temp)
                stack.append(s[i])
            else:
                stack.append(s[i])
                
            i+=1
        temp = ""
        while(stack and stack[-1] not in ['+','-','*','/']):
            temp =  stack.pop() + temp
        stack.append(temp)
                
        t = []
        
        i=0
        while(i<len(stack)):
            if stack[i] not in ['/','*']:
                t.append(stack[i])
            else:
                a = t.pop()
                b = stack[i+1]
                if stack[i] == '/':
                    res = int(a)//int(b)
                else:
                    res = int(a)*int(b)
                t.append(res)
                i+=1
            i+=1

        res = int(t[0])
        for i in range(1,len(t),2):
            print(t[i])
            if t[i] == '+':
                res+=int(t[i+1])
            else:
                res-=int(t[i+1])
                
        return res
                
            
24700