<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> operant;
        stack<char> operato;
        string curNum;
        for (char c : s) {
            if (c >= '0' && c <= '9') {
                curNum.push_back(c);
            } else if (c == ' ') {
                continue;
            } else {
                int right = stoi(curNum);
                curNum = "";
                // process * and / 
                if (!operato.empty()) {
                    char op = operato.top();
                    if (op == '*') {
                        int left = operant.top();
                        operant.pop();
                        right = left * right;
                        operato.pop();
                    } else if (op == '/') {
                        int left = operant.top();
                        operant.pop();
                        right = left / right;
                        operato.pop();
                    }
                }
                operant.push(right);
                operato.push(c);
            }
        }
        operant.push(stoi(curNum));
        if (!operato.empty()){
            char op = operato.top();
            if (op == '*' || op == '/') {
                operato.pop();
                int right = operant.top();
                operant.pop();
                int left = operant.top();
                operant.pop();
                if (op == '*') operant.push(left * right);
                else operant.push(left / right);
            }
        }
        stack<int> opr;
        stack<char> opt;
        while (!operato.empty()) {
            opt.push(operato.top());
            operato.pop();
        }
        while (!operant.empty()) {
            opr.push(operant.top());
            operant.pop();
        }
        while(!opt.empty()) {
            int left = opr.top();
            opr.pop();
            int right = opr.top();
            opr.pop();
            char op = opt.top();
            opt.pop();
            if (op == '+') {
                opr.push(left + right);
            } else if (op == '-') {
                opr.push(left - right);
            } else if (op == '*') {
                opr.push(left * right);
            } else if (op == '/') {
                opr.push(left / right);
            }
        }
        return opr.top();
    }
};
18268