evaluate-reverse-polish-notation

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Algorithms

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>.

Evaluate the expression. Return an integer that represents the value of the expression.

Note that:

<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 truncates toward zero.</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 32-bit integer.</li>
</ul>

&nbsp;
Example 1:

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

Example 2:

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

Example 3:

<pre>
Input: 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;]
Output: 22
Explanation: ((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>

&nbsp;
Constraints:

<ul>
	<li><code>1 &lt;= tokens.length &lt;= 104</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>