<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 {
    struct Element {
        bool is_operator;
        union {
            char _operator;
            unsigned long _operand;
        };
    };

    list<Element>::iterator perform_op(list<Element> & expression, list<Element>::iterator iter) {
        unsigned long new_operand;
        if (iter->_operator == '*') {
            new_operand = prev(iter)->_operand * next(iter)->_operand;
        } else if (iter->_operator == '/') {
            new_operand = prev(iter)->_operand / next(iter)->_operand;
        } else if (iter->_operator == '+') {
            new_operand = prev(iter)->_operand + next(iter)->_operand;
        } else if (iter->_operator == '-') {
            new_operand = prev(iter)->_operand - next(iter)->_operand;
        }
        --iter;
        expression.erase(next(iter, 2));
        expression.erase(next(iter));
        iter->_operand = new_operand;
        return iter;
    }

public:
    int calculate(string s) {
        list<Element> expression;
        string num;
        for (int i = 0; i < s.size(); ++i) {
            if (s[i] >= '0' && s[i] <= '9') {
                num += s[i];
            } else {
                if (!num.empty()) {
                    // cout << "pushing " << num << '\n';
                    expression.push_back({.is_operator = false, ._operand = stoul(num)});
                    num.clear();
                }

                if (s[i] != ' ') {
                    expression.push_back({.is_operator = true, ._operator = s[i]});
                }
            }
        }
        if (!num.empty()) {
            // cout << "pushing " << num << '\n';
            expression.push_back({.is_operator = false, ._operand = stoul(num)});
            num.clear();
        }

        // for (int i = 0; i < expression.size(); ++i) {
        //     if (expression[i].is_operator) {
        //         cout << expression[i]._operator;
        //     } else {
        //         cout << expression[i]._operand;
        //     }
        // }
        // cout << '\n';

        // multiplication and division pass
        for (auto iter = expression.begin(); iter != expression.end(); ++iter) {
            if (iter->is_operator && (iter->_operator == '*' || iter->_operator == '/')) {
                iter = perform_op(expression, iter);
            }
        }

        // for (int i = 0; i < expression.size(); ++i) {
        //     if (expression[i].is_operator) {
        //         cout << expression[i]._operator;
        //     } else {
        //         cout << expression[i]._operand;
        //     }
        // }
        // cout << '\n';

        // addition and subtraction pass
        for (auto iter = expression.begin(); iter != expression.end(); ++iter) {
            if (iter->is_operator && (iter->_operator == '+' || iter->_operator == '-')) {
                iter = perform_op(expression, iter);
            }
        }

        // for (int i = 0; i < expression.size(); ++i) {
        //     if (expression[i].is_operator) {
        //         cout << expression[i]._operator;
        //     } else {
        //         cout << expression[i]._operand;
        //     }
        // }
        // cout << '\n';

        return expression.front()._operand;
    }
};
39369