<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 s="";
    stack<int> number;
    stack<char> operators;

    void process(string ss){
        for(char i:ss){
            if(i!=' ')s+=i;
        }
    }

    void popp(){
        int t2=number.top();
        number.pop();
        int t1=number.top();
        number.pop();
        if(operators.top()=='*')number.push(t1*t2);
        if(operators.top()=='/')number.push(t1/t2);
        operators.pop();
    }  

    int calculate(string ss) {
        process(ss);
        int j;

        for(int i=0;i<s.size();i++){
            j=i;
            if(s[i]=='/'){
                operators.push('/');
            }
            else if(s[i]=='*'){
                operators.push('*');
            }
            else if(s[i]=='+'){
                while(operators.size() && (operators.top()=='/' || operators.top()=='*')){
                    popp();
                }
                operators.push('+');
            }
            // if the number has - before it we dont perform subtration
            // instead we make the number -ve and put it into number
            // and in operators we put + instead
            else if(s[i]=='-'){
                i++;j=i;
                while(operators.size() && (operators.top()=='/' || operators.top()=='*')){
                    popp();
                }
                while(i+1<s.size() && isdigit(s[i+1]))i++;
                cout<<(s.substr(j,i-j+1))<<" ";
                number.push(-1*stoi(s.substr(j,i-j+1)));

                if( operators.size() && (operators.top()=='/' || operators.top()=='*'))popp();

                operators.push('+');
            }else{
                while(i+1<s.size() && isdigit(s[i+1]))i++;
                cout<<(s.substr(j,i-j+1))<<" ";
                number.push(stoi(s.substr(j,i-j+1)));

                if( operators.size() && (operators.top()=='/' || operators.top()=='*'))popp();

            }
        }

        while(number.size()>1){
            if(operators.top()=='+'){
                int t2=number.top();
                number.pop();
                int t1=number.top();
                number.pop();
                number.push(t1+t2);
            }
            else if(operators.top()=='-'){
                int t2=number.top();
                number.pop();
                int t1=number.top();
                number.pop();
                number.push(t1-t2);
            }
            else if(operators.top()=='*'){
                int t2=number.top();
                number.pop();
                int t1=number.top();
                number.pop();
                number.push(t1*t2);
            }
            else{
                int t2=number.top();
                number.pop();
                int t1=number.top();
                number.pop();
                number.push(t1/t2);
            }
            operators.pop();
        }
        return number.top();
    }
};
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