<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:
    enum class op_t {
        ADD, SUB, MUL, DIV, VAL
    };

    struct ele_t {
        op_t op;
        int val;
    };

    int calculate(string s) {
        vector<ele_t> v;
        string curr;
        for (char c : s) {
            if (c == ' ') continue;

            if (isdigit(c)) {
                curr += c;
            } else {
                v.push_back((ele_t) { .op = op_t::VAL, .val = stoi(curr) });
                curr = "";
                switch (c) {
                    case '+': 
                        v.push_back((ele_t) { .op = op_t::ADD, .val = 0 });
                        break;
                    case '-': 
                        v.push_back((ele_t) { .op = op_t::SUB, .val = 0 });
                        break;
                    case '*': 
                        v.push_back((ele_t) { .op = op_t::MUL, .val = 0 });
                        break;
                    default: 
                        v.push_back((ele_t) { .op = op_t::DIV, .val = 0 });
                        break;
                }
            }
        }
        if (curr != "") {
            v.push_back((ele_t) { .op = op_t::VAL, .val = stoi(curr) });
        }

        vector<int> tmp = { v[0].val };
        int i = 1;
        while (i < v.size()) {
            op_t op = v[i].op;
            int right = v[i + 1].val;
            int left = tmp.back();
            switch (op) {
                case op_t::ADD:
                    tmp.push_back(right);
                    break;
                case op_t::SUB:
                    tmp.push_back(-right);
                    break;
                case op_t::MUL:
                    tmp.pop_back();
                    tmp.push_back(left * right);
                    break;
                default:
                    tmp.pop_back();
                    tmp.push_back(left / right);
                    break;
            }
            i += 2;
        }
        int ans = 0;
        for (auto v : tmp) ans += v;
        return ans;
        
    }
};
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