<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>

// 1. remove spaces
// 2. use a stack, 

// 3+2*2

// 3+2*
// for i from 0 to s.length() - 1:
//     if s[i] is number:
//         get the num
//         if !stk.empty() && stk.top() is '*' or '/':
//             pop two and get the results and push back
//     else if s[i] is operator:
//         push into stack

// number,op,number,op,number
// seperate operator stack and number stack

class Solution {
public:
    int calculate(string s2) {
        string s;
        for (char c : s2) {
            if (c != ' ')
                s.push_back(c);
        }
        stack<int> nums;
        int sign = 1;
        char prevOp = '0';
        for (int i = 0; i < s.length(); i++) {
            int num = 0;
            while (i < s.length() && s[i] >= '0' && s[i] <= '9') {
                num = num * 10 + (s[i] - '0');
                i++;
            }
            if (prevOp == '*' || prevOp == '/') {
                int num2 = nums.top();
                nums.pop();

                if (prevOp == '*')
                    nums.push(num2 * num);
                else
                    nums.push(num2 / num);
            }
            else // push '+' or '-'
                nums.push(num * sign);
            if (s[i] == '+')
                sign = 1;
            else if (s[i] == '-')
                sign = -1;
            if (i != s.length())
                prevOp = s[i];
        }
        while (nums.size() > 1) {
            int num = nums.top();
            nums.pop();
            int num2 = nums.top();
            nums.pop();
            nums.push(num + num2);
        }
        return nums.top();
    }
};
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