<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 {
    inline bool isOp(char c) {
        return (c == '+') || (c == '-') || (c == '/') || (c == '*');
    }
public:
    int calculate(const string& s) {
        int total { 0 };
        int exp { 0 };
        char lastOp { '+' };
        int currNum { 0 };

        for (auto& c : s) {
            if (c == ' ') continue;
            if (c >= '0' && c <= '9') {
                currNum = currNum * 10 + (c - '0');
            } else if (isOp(c)) {
                if (lastOp == '+') {
                    total += exp;
                    exp = currNum;
                } else if (lastOp == '-') {
                    total += exp;
                    exp = -currNum;
                } else if (lastOp == '*') {
                    exp *= currNum;
                } else {
                    exp /= currNum;
                }
                currNum = 0;
                lastOp = c;
            }
        }
        if (lastOp == '+') {
            exp += currNum;
        } else if (lastOp == '-') {
            exp -= currNum;
        } else if (lastOp == '*') {
            exp *= currNum;
        } else {
            exp /= currNum;
        }
        total += exp;
        return total;
    }
};
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