<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>

int stoi(string &s) {
    int ret = 0;
    for(auto i: s) {
        ret = ret*10 + (i-'0');
    }
    return ret;
}

class Solution {
public:
    int calculate(string s) {
        string p;
        deque<string> st;
        for(auto i: s) {
            if(i>='0' && i<='9') p.push_back(i);
            else if(i=='+' || i=='-' || i=='*' || i=='/') {
                if(st.size() && (st.back()=="*" || st.back()=="/")) {
                    string temp = st.back();
                    st.pop_back();
                    string val = st.back();
                    st.pop_back();
                    if(temp == "*") st.push_back(to_string(stoi(val) * stoi(p)));
                    else st.push_back(to_string(stoi(val) / stoi(p)));
                }
                else {
                    st.push_back(p);
                }
                p="";
                p.push_back(i);
                st.push_back(p);
                p="";
            }
        }
        if(st.size() && (st.back()=="*" || st.back()=="/")) {
            string temp = st.back();
            st.pop_back();
            string val = st.back();
            st.pop_back();
            if(temp == "*") st.push_back(to_string(stoi(val) * stoi(p)));
            else st.push_back(to_string(stoi(val) / stoi(p)));
        }
        else st.push_back(p);
        int val = stoi(st.front());
        st.pop_front();
        while(st.size()) {
            string op = st.front();
            st.pop_front();
            int v = stoi(st.front());
            st.pop_front();
            if(op=="+") val += v;
            else if(op=="-") val -= v;
            else if(op=="*") val*=v;
            else val /= v;
        }
        return val;
    }
};
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