<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<ele_t> v1;
        int i = 0;
        while (i < v.size()) {
            if (v[i].op != op_t::MUL && v[i].op != op_t::DIV) {
                v1.push_back(v[i]);
                ++i;
            } else if (v[i].op == op_t::MUL) {
                int result = v1.back().val * v[i + 1].val;
                v1.pop_back();
                v1.push_back((ele_t) { .op = op_t::VAL, .val = result });
                i += 2;
            } else {
                int result = v1.back().val / v[i + 1].val;
                v1.pop_back();
                v1.push_back((ele_t) { .op = op_t::VAL, .val = result });
                i += 2;
            }
        }

        if (v1.size() == 1) return v1[0].val;

        int ans = v1[0].val;
        for (int i = 1; i + 1 < v1.size(); i += 2) {
            int num1 = v1[i + 1].val;
            if (v1[i].op == op_t::ADD) {
                ans += num1;
            } else {
                ans -= num1;
            }
        }
        return ans;
        
    }
};
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