<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 {
    std::vector<int> stack;

    void updateStack(int number, char operation) {
        if (operation == '*') {
            stack.back() *= number;
        }
        else if (operation == '/') {
            stack.back() /= number;
        } else {
            stack.push_back(number);
        }      
    }

public:
    int calculate(string s) {
        bool isNumber = false;
        int sign = 1;
        int number = 0;
        char operation = 0;

        for (auto c : s) {
            if (c == ' ' || c == '+' || c == '-' || c == '*' || c == '/') {
                if (isNumber) {
                    updateStack(number, operation);
                    sign = 1;
                    operation = 0;
                    number = 0;
                    isNumber = false;
                }
                if (c == '-') {
                    sign *= -1;
                }
                else if (c == '*' || c == '/') {
                    operation = c;
                }
            }
            else if (c >= '0' && c <= '9') { //digit
                isNumber = true;
                number = 10*number + sign*( (int)(c) - int('0') );
            }
        }
        if (isNumber) {
            updateStack(number, operation);
        }
        int result = 0;
        for (auto num: stack) {
            result += num;
        }
        return result; 
    }
};
15371