leetcode-problemset/算法题（国外版）/evaluate-reverse-polish-notation.html

<p>Evaluate the value of an arithmetic expression in <a href="http://en.wikipedia.org/wiki/Reverse_Polish_notation" target="_blank">Reverse Polish Notation</a>.</p>

<p>Valid operators are <code>+</code>, <code>-</code>, <code>*</code>, and <code>/</code>. Each operand may be an integer or another expression.</p>

<p><strong>Note</strong> that division between two integers should truncate toward zero.</p>

<p>It is guaranteed that the given RPN expression is always valid. That means the expression would always evaluate to a result, and there will not be any division by zero operation.</p>

<p>&nbsp;</p>
<p><strong>Example 1:</strong></p>

<pre>
<strong>Input:</strong> tokens = [&quot;2&quot;,&quot;1&quot;,&quot;+&quot;,&quot;3&quot;,&quot;*&quot;]
<strong>Output:</strong> 9
<strong>Explanation:</strong> ((2 + 1) * 3) = 9
</pre>

<p><strong>Example 2:</strong></p>

<pre>
<strong>Input:</strong> tokens = [&quot;4&quot;,&quot;13&quot;,&quot;5&quot;,&quot;/&quot;,&quot;+&quot;]
<strong>Output:</strong> 6
<strong>Explanation:</strong> (4 + (13 / 5)) = 6
</pre>

<p><strong>Example 3:</strong></p>

<pre>
<strong>Input:</strong> tokens = [&quot;10&quot;,&quot;6&quot;,&quot;9&quot;,&quot;3&quot;,&quot;+&quot;,&quot;-11&quot;,&quot;*&quot;,&quot;/&quot;,&quot;*&quot;,&quot;17&quot;,&quot;+&quot;,&quot;5&quot;,&quot;+&quot;]
<strong>Output:</strong> 22
<strong>Explanation:</strong> ((10 * (6 / ((9 + 3) * -11))) + 17) + 5
= ((10 * (6 / (12 * -11))) + 17) + 5
= ((10 * (6 / -132)) + 17) + 5
= ((10 * 0) + 17) + 5
= (0 + 17) + 5
= 17 + 5
= 22
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= tokens.length &lt;= 10<sup>4</sup></code></li>
	<li><code>tokens[i]</code> is either an operator: <code>&quot;+&quot;</code>, <code>&quot;-&quot;</code>, <code>&quot;*&quot;</code>, or <code>&quot;/&quot;</code>, or an integer in the range <code>[-200, 200]</code>.</li>
</ul>