<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 evaluate(int a, int b, char op){
        if(op == '+')
            return a;
        else if(op == '-')
            return -a;
        else if(op == '*')
            return a*b;
        else
            return a/b;
    }

    int calculate(string s) {
        stack<int> nums;
        int i = 0;
        s.push_back('@');
        int n = s.size();
        int cur = 0;
        char op = '+';
        while(i<n){
            if(s[i] == ' '){
                i++;
            } else if(s[i] >='0' && s[i] <='9' ){
                string numstr = "";
                while(i<n && s[i] >='0' && s[i] <='9'){
                    numstr.push_back(s[i++]);
                }
                cur = stoi(numstr);
            } else {
                if(op == '*'||op == '/'){
                    int v = nums.top(); nums.pop();
                    nums.push(evaluate(v, cur, op));
                } else {
                    nums.push(evaluate(cur, 0, op));
                }

                op = s[i++];
                cur = 0;
            }
        }

        int res = 0;
        while(!nums.empty()){
            res += nums.top();
            nums.pop();
        }
        return res;
    }
};
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