<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>

#define ADD "+"
#define SUB "-"
#define MUL "*"
#define DIV "/"
class Solution {
public:
    int calculate(string A) {
        stack<string> S;
        int it = 0;
        int n = A.size();
        bool neg = false;
        while (it < n) {
            string cur(1, A[it]); 
            if (cur == " ") {
                it++;
                continue;
            }
            if (cur == ADD || cur == SUB || cur == MUL || cur == DIV) {
                assert(!S.empty());
                if (cur == SUB) {
                    cur = ADD;
                    neg = true;
                } 
                S.push(cur);
                it++;
                continue;
            } else {
                string op = "";
                while (it < n && A[it] >= '0' && A[it] <= '9') {
                    op += A[it];
                    it++;
                }
                // cout << it;
                if (op != "") {
                    if (!S.empty() && (S.top() == MUL || S.top() == DIV)) {
                        int op1 = stoi(op);
                        string oper = S.top();
                        S.pop();
                        int op2 = stoi(S.top());
                        S.pop();
                        int res = oper == DIV ? op2/op1 : op1*op2; 
                        S.push(to_string(res));
                    } else {
                        string tmp;
                        if (neg) {
                            tmp = "-";
                            tmp += op;
                        }
                        neg ? S.push(tmp) : S.push(op);
                        neg = false;
                    }
                }
            }
        }
        while (S.size()>1) {
            int res = 0;
            int op1 = stoi(S.top());
            cout << "op1: " << op1; 
            S.pop();
            string op = S.top();
            cout << ", op: " << op;
            S.pop();
            int op2 = stoi(S.top());
            cout << ", op2: " << op2; 
            S.pop();
            res = op1+op2;
            S.push(to_string(res));
        }
        // cout << S.top();
        return stoi(S.top());
    }
};
36059