<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) {
        int i = 0;
        int acc = 0;
        string num = "";
        while(i < s.size() && !isOperator(s[i])) {
            if (s[i] != ' ') num += s[i];
            i++;
        }
        int prev = stoi(num);

        while (i < s.size()) {
            char opr = s[i];
            i++;
            num = "";
            while(i < s.size() && !isOperator(s[i])) {
                if (s[i] != ' ') num += s[i];
                i++;
            }


            if (opr == '*') {
                prev *= stoi(num);
            } else if (opr == '/') {
                prev /= stoi(num);
            } else if (opr == '+') {
                acc += prev;
                prev = stoi(num);
            } else {
                acc += prev;
                prev = stoi(num)*-1;
            }

        }
    
        return acc + prev;
    }

    bool isOperator(char c) {
        return c == '*' || c == '/' || c == '+' || c == '-';
    }
};
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