<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) {
        string str = "";
        for(auto it: s) {
            if(it != ' ') str += it;
        }
        str += '/';
        deque<string> dq;
        int start = 0;
        string currOp = " ";

        for(int i=0; i<str.size(); i++) {
            char it = str[i];
            if(it == '+' || it == '-' || it == '*' || it == '/') {
                string sub = str.substr(start, i - start);
                dq.push_back(sub);
                if(currOp == "*" || currOp == "/") {
                    int n2 = stoi(dq.back());
                    dq.pop_back();
                    dq.pop_back();
                    int n1 = stoi(dq.back());
                    dq.pop_back();
                    if(currOp == "*") dq.push_back(to_string((int)(n1 * n2)));
                    else dq.push_back(to_string((int)(n1 / n2)));
                }
                currOp.assign(1, it);
                start = i+1;
                dq.push_back(currOp);
            }
        }
        dq.pop_back();

        while(dq.size() != 1) {
            int n1 = stoi(dq.front());
            dq.pop_front();
            string op = dq.front();
            dq.pop_front();
            int n2 = stoi(dq.front());
            dq.pop_front();
            if(op == "+") dq.push_front(to_string(n1+n2));
            else dq.push_front(to_string(n1-n2));
        }

        return stoi(dq.front());
    }
};
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