<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 solve(int a,int b,char c){
        cout<<a<<" "<<b<<" "<<c<<endl;
        if(c=='+') return a+b;
        if(c=='-') return b-a;
        if(c=='*') return a*b;
        if(c=='/') return b/a;
        return 0;
    }
    int prio(char st){
        if(st=='+'||st=='-'){
            return 1;
        }
        else if(st=='*' || st=='/'){
            return 2;
        }
        else return 0;
    }
    int calculate(string s) {
        stack<int> nums;
        stack<char> op;
        int n=s.length();
        int newnum=0;
        for(int i=0;i<n;i++){
            if(s[i]>=48 && s[i]<=57){
                int x=s[i]-48;
                if(nums.empty()||newnum) nums.push(x);
                else{
                    x=(nums.top()*10)+x;
                    nums.pop();
                    nums.push(x);
                }
                newnum=0;
            }
            else if(s[i]==' ') continue;
            else{
                newnum=1;
                if(op.empty()){
                    op.push(s[i]);
                }
                else if(prio(s[i])>prio(op.top())){
                    op.push(s[i]);
                }
                else{
                    while((op.size()>0) && (prio(op.top())>=prio(s[i]))){
                        char oper=op.top();
                        op.pop();
                        int a=nums.top();
                        nums.pop();
                        int b=nums.top();
                        nums.pop();
                        int vals=solve(a,b,oper);
                        nums.push(vals);
                    }
                    op.push(s[i]);
                }
            }
        }
        while(op.size()!=0){
            char oper=op.top();
            op.pop();
            int a=nums.top();
            nums.pop();
            int b=nums.top();
            nums.pop();
            int vals=solve(a,b,oper);
            nums.push(vals);
        }
        return nums.top();
    }
};
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