<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 t;
        int pre=0;
        for(int i=0;i<s.size();i++){
            if(s[i]==' ')continue;
            t+=s[i];
            
        }
        cout<<t<<endl;
        // stack<int>st;
        vector<long long int>v;
        for(int i=0;i<t.size();){
            if(t[i]=='*'){v.push_back(1);i++;}
            else if(t[i]=='+'){v.push_back(2);i++;}
            else if(t[i]=='/'){v.push_back(-1);i++;}
            else if(t[i]=='-'){v.push_back(-2); i++;}
            else {
                long long int ctn=0;
                while(i<t.size()&&t[i]>='0'&&t[i]<='9'){
                    ctn=ctn*10+t[i]-'0';
                    i++;
                }
                // i--;
                v.push_back(ctn);
            }
        }
        for(auto i:v)cout<<i<<" ";
        // ////
        for(int i=1;i<v.size();){
            if(v[i]==-1){
                long long int c=v[i-1]/v[i+1];
                v[i-1]=c;
                v.erase(v.begin()+i);
                v.erase(v.begin()+i);
            }
            else if(v[i]==1){
                long long int c=v[i-1]*v[i+1];
                v[i-1]=c;
                v.erase(v.begin()+i);
                v.erase(v.begin()+i);
            }
            else i=i+2;
        }
        // for(int i=1;i<v.size();){
            
        //     else i=i+2;
        // }
        // cout<<endl;
        // for(auto i:v)cout<<i<<" ";
        // for(int i=1;i<v.size();){
        //     if(v[i]==2){
        //         long long int c=v[i-1]+v[i+1];
        //         v[i-1]=c;
        //         cout<<c<<" 8888 "<<endl;
        //         v.erase(v.begin()+i);
        //         v.erase(v.begin()+i);
        //     }
        //     else i=i+2;
        // }
        // cout<<endl;
        // for(auto i:v)cout<<i<<" ";
        // for(int i=1;i<v.size();){
        //     if(v[i]==-2){
        //         long long int c=(v[i-1]-v[i+1]);
        //         v[i-1]=c;
        //         v.erase(v.begin()+i);
        //         v.erase(v.begin()+i);
        //     }
        //     else i=i+2;
        // }
        // return v[0];
        long long int ans=0;
        for(int i=1;i<v.size();i=i+2){
            if(v[i]==2)v[i+1]=v[i+1];
            else if(v[i]==-2)v[i+1]=-v[i+1];
        }
        for(int i=0;i<v.size();i=i+2)ans+=v[i];
        return ans;

    }
};
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