# 0150-evaluate-reverse-polish-notation


Try it on <a href='https://leetcode.com/problems/0150-evaluate-reverse-polish-notation'>leetcode</a>

## Description
<div class="description">
<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>Evaluate the expression. Return <em>an integer that represents the value of the expression</em>.</p>

<p><strong>Note</strong> that:</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 class="example">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 class="example">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 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;]
<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>

</div>

## Solution(Python)
```Python
class Solution:
    # Time complexity: O(n)
    # Space complexity: O(n)
    def evalRPN(self, tokens: List[str]) -> int:
        operations = {
            "+": lambda a,b: a+b,
            "-": lambda a,b: a-b,
            "*": lambda a,b: a*b,
            "/": lambda a,b: int(a/b),
        }
        stack = []

        for token in tokens:
            if token in operations:
                num2 = stack.pop()
                num1 = stack.pop()
                operation = operations[token]
                stack.append(operation(num1, num2))
            else:
                stack.append(int(token))
        return stack.pop()
```