<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
        N = len(s)
        curr = -1
        curr_op = ""
        while i < N:            
            if s[i].isdigit():
                n_str = ""
                while i < N and s[i].isdigit():
                    n_str += s[i]
                    i += 1
                curr = int(n_str)
                i -= 1
            elif s[i] == "-":
                curr_op = "-"
            elif s[i] == "*":
                curr_op = "*"
            elif s[i] == "/":
                curr_op = "/"     

            if curr_op == "*" and curr >= 0:
                prev = stack.pop()
                res = prev * curr
                stack.append(res)
                curr_op = ""
                curr = -1
            elif curr_op == "/" and curr >= 0:
                prev = stack.pop()
                res = int(prev / curr)
                stack.append(res)
                curr_op = ""
                curr = -1
            elif curr_op == "-" and curr >= 0:
                stack.append(-1 * curr) 
                curr_op = ""                     
                curr = -1               
            elif curr >= 0:
                stack.append(curr)                      
                curr = -1

            i += 1

        print(stack)
        result = 0
        while stack:
            result += stack.pop()
        return result           
    
20700