<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 calculate(string s) {
        stack<int> s1;
        stack<char> s2;
        int i = 0;
        while (i < s.size()) {
            if (s[i] == ' ') {
            } else if (isdigit(s[i])) {
                // number
                int num = int(s[i]) - 48;
                i++;
                while (isdigit(s[i])) {
                    num = num * 10;
                    num = num + (int(s[i]) - 48);
                    i++;
                }
                if (s2.size() != 0 && s2.top() == '-') {
                    s1.push(-num);
                } else {
                    s1.push(num);
                }
                continue;
            } else {
                // operator
                if (!s2.empty() && s2.top() == '/') {
                    int n1 = s1.top();
                    s1.pop();
                    int n2 = s1.top();
                    s1.pop();
                    s1.push(n2 / n1);
                    s2.pop();
                } else if (!s2.empty() && s2.top() == '*') {
                    int n1 = s1.top();
                    s1.pop();
                    int n2 = s1.top();
                    s1.pop();
                    s1.push(n2 * n1);
                    s2.pop();
                }
                s2.push(s[i]);
            }
            i++;
        }
        while (s2.size() != 0) {
            if (!s2.empty() && s2.top() == '/') {
                int n1 = s1.top();
                s1.pop();
                int n2 = s1.top();
                s1.pop();
                s1.push(n2 / n1);
                s2.pop();
            } else if (!s2.empty() && s2.top() == '*') {
                int n1 = s1.top();
                s1.pop();
                int n2 = s1.top();
                s1.pop();
                s1.push(n2 * n1);
                s2.pop();
            } 
            // else if (!s2.empty() && s2.top() == '-') {
            //     int n1 = s1.top();
            //     s1.pop();
            //     int n2 = s1.top();
            //     s1.pop();
            //     if (n1 < 0) {
            //         s1.push(n1 - n2);
            //     } else {
            //         s1.push(n2 - n1);
            //     }

            //     s2.pop();
            // }
             else {
                int n1 = s1.top();
                s1.pop();
                int n2 = s1.top();
                s1.pop();
                s1.push(n2 + n1);
                s2.pop();
            }
        }
        return s1.top();
    }
};
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