<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:
        # s = "3         +           2        *            2"
        # stack[-1]. last_operation pre_num cur_operation cur_num
        stack = []
        last_operation = ''
        pre_num = 0
        cur_operation = ''
        cur_num = 0
        flag = 1

        for i in s:
            if i.isdigit():
                cur_num = pre_num*10 + int(i)
                pre_num = cur_num

            elif i in '+-*/':
                if last_operation:
                    if last_operation == '+':
                        stack.append(cur_num)
                        pre_num = 0
                        cur_num = 0
                    elif last_operation == '-':
                        stack.append(flag*cur_num)
                        pre_num = 0
                        cur_num = 0
                        flag = 1
                    elif last_operation == '*':
                        stack.append(stack.pop()*cur_num)
                        pre_num = 0
                        cur_num = 0
                    elif last_operation == '/':
                        if stack[-1] > 0:
                            stack.append(stack.pop()//cur_num)
                        else:
                            stack.append(ceil(stack.pop()/cur_num))
                        pre_num = 0
                        cur_num = 0
                    if i == '+':
                        last_operation = '+'
                    elif i == '-':
                        last_operation = '-'
                        flag = -1
                    elif i == '*':
                        last_operation = '*'
                    elif i == '/':
                        last_operation = '/'
                else:
                    if i == '+':
                        stack.append(cur_num)
                        pre_num = 0
                        cur_num = 0
                        last_operation = '+'
                    elif i == '-':
                        stack.append(cur_num)
                        pre_num = 0
                        cur_num = 0
                        flag = -1
                        last_operation = '-'
                    elif i == '*':
                        stack.append(cur_num)
                        pre_num = 0
                        cur_num = 0
                        last_operation = '*'
                    elif i == '/':
                        stack.append(cur_num)
                        pre_num = 0
                        cur_num = 0
                        last_operation = '/'
        else:
            if last_operation == '+':
                stack.append(cur_num)
            elif last_operation == '-':
                stack.append(flag*cur_num)
            elif last_operation == '*':
                stack.append(stack.pop()*cur_num)
            elif last_operation == '/':
                if stack[-1] > 0:
                    stack.append(stack.pop()//cur_num)
                else:
                    stack.append(ceil(stack.pop()/cur_num))
        print(stack)
        if stack:
            return sum(stack)
        else:
            return cur_num



        
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