<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:

    string remove_space(string s){
        string tmp = "";
        for(char c: s){
            if(c != ' ') tmp += c;
        }
        return tmp;
    }

    int calculate(string s) {
        
        stack<string> st;
        
        int i = 0;

        s = remove_space(s);
        int n = s.length();

        while(i < n){
            if(s[i] == '*' || s[i] == '/'){
                cout<<st.top()<<"\n";
                int lhs = stoi(st.top());
                st.pop();
                int rhs = 0;
                string tmp = "";
                bool mult = (s[i] == '*')? true : false;
                i++;
                while(i < n && (s[i] >= '0' && s[i] <= '9')){
                    tmp += s[i++];
                }
                rhs = stoi(tmp);
                int ans = 0;
                if (mult) ans = lhs * rhs; 
                else ans = lhs/rhs;
                st.push(to_string(ans));
            }
            else if (s[i] == '+' || s[i] == '-'){
                // string a = "";
                // a += s[i];
                // st.push(a);
                bool add = (s[i] == '+')? true : false;
                i++;
                string tmp = "";
                while(i < n && (s[i] >= '0' && s[i] <= '9')){
                    tmp += s[i++];
                }
                if(add) st.push(tmp);
                else{
                    int num = -1 * stoi(tmp);
                    st.push(to_string(num));
                }
            }
            else if(s[i] >= '0' && s[i] <= '9'){
                string tmp = "";
                while(i < n && (s[i] >= '0' && s[i] <= '9')){
                    tmp += s[i++];
                }
                // int num = stoi(tmp);
                st.push(tmp);
            }
        }

        int ans = 0;
        bool add = false;
        bool start = true;
        while(!st.empty()){
            string top = st.top();
            ans += stoi(top);
            st.pop();
        }
        return ans;

    }
};
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