<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:
    bool check(string i) {
        if(i != "+" && i != "-" && i != "*" && i != "/") return true;
        return false;
    }
    void resolve(deque<string>& st) {
        string t = st.back();
        st.pop_back();
        string op = st.back();
        st.pop_back();
        string prev = st.back();
        st.pop_back();
        if(op == "*") {
            st.push_back(to_string(stoi(prev) * stoi(t)));
        }
        else if(op == "/") {
            st.push_back(to_string(stoi(prev) / stoi(t)));
        }
        else{
            st.push_back(prev); st.push_back(op); st.push_back(t);
        }
    }
    int calculate(string s) {
        deque<string> st;
        bool fin = false;
        for(auto &ch:s) {
            string i = "";
            i += ch;
            // cout<<i<<endl;
            if(i == " ") continue;
            if(st.empty()){
                st.push_back(i);
                continue;
            }
            if(!check(i)) {
                if(fin) {
                    resolve(st);
                    fin = false;
                }
                if(i == "*" || i == "/") fin = true;
                st.push_back(i);
            }
            else {
                string t = st.back();
                if(check(t)) {
                    st.pop_back();
                    st.push_back(to_string(stoi(t) * 10 + stoi(i)));
                }
                else st.push_back(i);
            }
            
        }
        if(st.size() >= 3) resolve(st);
        int ans = stoi(st.front());
        // cout<<ans<<endl;
        string op;
        st.pop_front();
        while(!st.empty()) {
            // cout<<st.front()<<endl;
            if(!check(st.front())) op = st.front();
            else{
                if(op == "+") ans += stoi(st.front());
                else if(op == "-") ans -= stoi(st.front());
                
            }
            st.pop_front();
        }
        return ans;
    }
};
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