<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) {
        s += 'e';
        vector<int> arr1;
        vector<char> arr2;
        int n = s.size();

        for (int i = 0; i < n; ++i){
            if (s[i] > '9' || s[i] < '0'){
                if (s[i] == '+' || s[i] == '-' || s[i] == '*' || s[i] == '/'){
                    arr2.emplace_back(s[i]);
                }
                continue;
            }
            int temp = 0;
            for (int j = i; j < n; ++j){
                if (s[j] > '9' || s[j] < '0'){
                    arr1.emplace_back(temp);
                    i = j;

                    if (s[j] == '+' || s[j] == '-' || s[j] == '*' || s[j] == '/'){
                        arr2.emplace_back(s[j]);
                    }
                    break;
                }else{
                    temp = temp * 10 - '0' + s[j];
                }
            }
        }

        stack<int> stk1;
        stack<char> stk2;

        stk1.emplace(arr1[0]);
        stk2.emplace('+');
        
        for (int i = 1; i < arr1.size(); ++i){
            stk1.emplace(arr1[i]);
            stk2.emplace(arr2[i-1]);
            if (stk2.top() == '*'){
                int x = stk1.top();
                stk1.pop();
                int y = stk1.top();
                stk1.pop();
                stk1.emplace(y * x);
                stk2.pop();
            }else if (stk2.top() == '/'){
                int x = stk1.top();
                stk1.pop();
                int y = stk1.top();
                stk1.pop();
                stk1.emplace(y / x);
                stk2.pop();
            }
        }

        int ans = 0;
        while(!stk2.empty()){
            if (stk2.top() == '+'){
                ans += stk1.top();
                stk2.pop();
                stk1.pop();
            }else{
                ans -= stk1.top();
                stk2.pop();
                stk1.pop();
            }
        }
        return ans;
    }
};
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