<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 calc(int value1, char oper, int value2){
        switch(oper){
            case '+':
            return value1 + value2;
            case '-':
            return value1 - value2;
            case '*':
            return value1 * value2;
            case '/':
            return value1 / value2;
        }
        return 0;
    }
    int calculate(string s) {
        string temp = "";
        for(int i=0;i<s.length();++i){
            if(s[i]!= ' ') temp += s[i];
        }
        s = temp;
        stack<int> numbers;
        stack<char> opers;
        string num = "";
        bool sign = false;
        for(int i=0;i<s.length();++i){
            if(isdigit(s[i])){
                num += s[i];
            }else{
                int temp = std::stoi(num);
                if(sign){
                    temp *= -1;
                    sign = false;
                }
                if(s[i] == '-') {
                    sign = true;
                    s[i] = '+';    
                }
                numbers.push(temp);
                num = "";
                
                if(!opers.empty() && (opers.top() == '*' || opers.top() == '/')){
                    int value2 = numbers.top();
                    numbers.pop();
                    int value1 = numbers.top();
                    numbers.pop();
                    char oper = opers.top();
                    opers.pop();
                    int result = calc(value1, oper, value2);
                    numbers.push(result);
                }
                opers.push(s[i]);
            }
        }
        int last = std::stoi(num);
        if(sign){
            last *= -1;
        }
        numbers.push(last);
        while(!opers.empty()){
            int value2 = numbers.top();
            numbers.pop();
            int value1 = numbers.top();
            numbers.pop();
            char oper = opers.top();
            opers.pop();
            int result = calc(value1, oper, value2);
            numbers.push(result);
        }
        return numbers.top();
    }
};
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