<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 {
public:
    int calculate(string s) {
        // operands are important
        // if it's + or - execute right away
        // if it's * or / gotta undo, operate, add.
        // remember to store negative previous by *-1
        int res = 0;
        int current = 0;
        int previous = 0;

        char op = '+';

        for(int idx = 0; idx < s.size(); idx++)
        {
            // check if we have a space
            if( s[idx] == ' ')
            {
                continue;
            }
            // check if we have a space
            if(isdigit(s[idx]))
            {
                string number;
                while((idx < s.size()) && isdigit(s[idx]))
                {
                    number += s[idx];
                    idx++;
                }
                // let outer loop handle the idx
                idx--;
                current = stoi(number);

                if(op == '+')
                {
                    res += current;
                    previous = current;
                } else if (op == '-')
                {
                    res -= current;
                    previous = current * -1;
                } else if (op == '/')
                {
                    // undo op first
                    res -= previous;
                    res += (previous / current);
                    previous = (previous / current);
                } else
                {
                    // undo op first
                    res -= previous;
                    res += (current * previous);
                    previous = current * previous;
                }
                current = 0;
            }
            else
            {
                op = s[idx];
            }
        }
        return res;        
    }
};
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