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leetcode-cn/problem (English)/逆波兰表达式求值(English) [evaluate-reverse-polish-notation].html
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<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>You are given an array of strings <code>tokens</code> that represents an arithmetic expression in a <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>Evaluate the expression. Return <em>an integer that represents the value of the expression</em>.</p>
<p><strong>Note</strong> that division between two integers should truncate toward zero.</p>
<p><strong>Note</strong> that:</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>
<ul>
<li>The valid operators are <code>&#39;+&#39;</code>, <code>&#39;-&#39;</code>, <code>&#39;*&#39;</code>, and <code>&#39;/&#39;</code>.</li>
<li>Each operand may be an integer or another expression.</li>
<li>The division between two integers always <strong>truncates toward zero</strong>.</li>
<li>There will not be any division by zero.</li>
<li>The input represents a valid arithmetic expression in a reverse polish notation.</li>
<li>The answer and all the intermediate calculations can be represented in a <strong>32-bit</strong> integer.</li>
</ul>
<p>&nbsp;</p>
<p><strong>Example 1:</strong></p>
<p><strong class="example">Example 1:</strong></p>
<pre>
<strong>Input:</strong> tokens = [&quot;2&quot;,&quot;1&quot;,&quot;+&quot;,&quot;3&quot;,&quot;*&quot;]
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<strong>Explanation:</strong> ((2 + 1) * 3) = 9
</pre>
<p><strong>Example 2:</strong></p>
<p><strong class="example">Example 2:</strong></p>
<pre>
<strong>Input:</strong> tokens = [&quot;4&quot;,&quot;13&quot;,&quot;5&quot;,&quot;/&quot;,&quot;+&quot;]
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<strong>Explanation:</strong> (4 + (13 / 5)) = 6
</pre>
<p><strong>Example 3:</strong></p>
<p><strong class="example">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;]