<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> stk;
        int num = 0;
        bool resolve = false;
        for (char c : s) {
            if (c >= '0' and c <= '9') {
                num = num * 10 + (c - '0');
            } else if (c == ' ') {
                continue;
            } else {
                stk.push(num);
                num = 0;
                if (resolve) {
                    int op2 = stk.top();
                    stk.pop();
                    int op = stk.top();
                    stk.pop();
                    int op1 = stk.top();
                    stk.pop();
                    cout << "::: " << op1 << " " << op << " " << op2 << endl;

                    if (op == -1) {
                        stk.push(op1 * op2);
                    } else {
                        stk.push(op1 / op2);
                    }
                    resolve = false;
                }

                if (c == '*') {
                    stk.push(-1);
                    resolve = true;
                } else if (c == '/') {
                    stk.push(-2);
                    resolve = true;
                } else if (c == '+') {
                    stk.push(-3);
                } else if (c == '-') {
                    stk.push(-4);
                } else {
                    cout << "unhandled" << c << endl;
                    return -1;
                }
            }
        }

        stk.push(num);
        num = 0;
        if (resolve) {
            int op2 = stk.top();
            stk.pop();
            int op = stk.top();
            stk.pop();
            int op1 = stk.top();
            stk.pop();
            cout << "::: " << op1 << " " << op << " " << op2 << endl;

            if (op == -1) {
                stk.push(op1 * op2);
            } else {
                stk.push(op1 / op2);
            }
            resolve = false;
        }
        vector<int> v;
        while(!stk.empty()){
            v.push_back(stk.top());
            stk.pop();
        }
        reverse(v.begin(), v.end());

        int res = v[0];

        for(int i = 1; i < v.size()-1; i+=2){
            int op = v[i], x = v[i+1];
            cout << res << " " << op << " " << x << endl;
            if(op == -3) res += x;
            else if(op == -4) res -= x;
            else cout << "unhandled" << endl;   
        }

        return res;
    }
};


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