<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 associativity(char cur){
    if(cur=='+' || cur=='-') return 1;
    if(cur=='*' || cur=='/') return 2;
    return 0;
  }
    string postFix(string &s){
        stack<char> st;
        st.push('&');
        string res="";
         int i=0,n=s.length();
         while(i<n){
            string t="";
            // if(s[i]==' '){
            //     i++;
            // }
            while(i<n && s[i]>='0' && s[i]<='9'){
                t+=s[i];i++;
            }
            if(t!=""){res+=t;res+='#'; continue;}
            else if(s[i]!=' '){
            while( associativity(st.top())>=associativity(s[i])){
                res+=st.top();st.pop();
            }
            st.push(s[i]);
            }
             i++;
          
            
        }
          while(!st.empty()){
              res+=st.top();
              st.pop();
            }
            res.pop_back();
            return res;

    }
    int compute(int a,int b,char c){
        if(c=='+') return a+b;
        if(c=='-') return a-b;
        if(c=='*') return a*b;
        if(c=='/') return a/b;
        return 0;
    }
    int calculate(string s) {
       string ans= postFix(s);
     //  cout<<ans<<" ";
      int i=0;
      int n=ans.size();
      stack<int> st;
      while(i<n){
        string num="";
        while(i<n && ans[i]>='0' && ans[i]<='9' ){
            num+=ans[i];
            i++;
        }
        if(num!=""){
            st.push(stoi(num));continue;
        }
        else if(ans[i]!='#'){
            int b=st.top();
            st.pop();
            int a=st.top();
            st.pop();
            int val=compute(a,b,ans[i]);
            st.push(val);
        }i++;
      }
       return st.top();
    }
};
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