<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:
    long calculate(string s) {
        char sign = '+';
        long num = 0;
        s+='+';
        stack<long> stack;
        for(char a : s){
            if(isdigit(a)) num = num*10 + a-'0';

            if(a=='+' || a=='-' || a=='*' || a=='/'){
                if(sign == '+') stack.push(num);
                if(sign == '-') stack.push(num*(-1));
                if(sign == '*'){
                    long prev = stack.top();
                    stack.pop();
                    stack.push(prev*num);
                }
                if(sign == '/'){
                    long prev = stack.top();
                    stack.pop();
                    stack.push(prev/num);
                }
                sign = a;
                num = 0;
            }
        }
        long res = 0;
        while(stack.size()){
            res += stack.top();
            stack.pop();
        }
        return res;
    }
};
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