# 941-valid-mountain-array


Try it on <a href='https://leetcode.com/problems/941-valid-mountain-array'>leetcode</a>

## Description
<div class="description">
<div><p>Given an array of integers <code>arr</code>, return <em><code>true</code> if and only if it is a valid mountain array</em>.</p>

<p>Recall that arr is a mountain array if and only if:</p>

<ul>
	<li><code>arr.length &gt;= 3</code></li>
	<li>There exists some <code>i</code> with <code>0 &lt; i &lt; arr.length - 1</code> such that:
	<ul>
		<li><code>arr[0] &lt; arr[1] &lt; ... &lt; arr[i - 1] &lt; arr[i] </code></li>
		<li><code>arr[i] &gt; arr[i + 1] &gt; ... &gt; arr[arr.length - 1]</code></li>
	</ul>
	</li>
</ul>
<img src="https://assets.leetcode.com/uploads/2019/10/20/hint_valid_mountain_array.png" width="500">
<p>&nbsp;</p>
<p><strong>Example 1:</strong></p>
<pre><strong>Input:</strong> arr = [2,1]
<strong>Output:</strong> false
</pre><p><strong>Example 2:</strong></p>
<pre><strong>Input:</strong> arr = [3,5,5]
<strong>Output:</strong> false
</pre><p><strong>Example 3:</strong></p>
<pre><strong>Input:</strong> arr = [0,3,2,1]
<strong>Output:</strong> true
</pre>
<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= arr.length &lt;= 10<sup>4</sup></code></li>
	<li><code>0 &lt;= arr[i] &lt;= 10<sup>4</sup></code></li>
</ul>
</div>
</div>

## Solution(Python)
```Python
class Solution:
    def validMountainArray(self, arr: List[int]) -> bool:
        n = len(arr)
        if n < 3:
            return False
        i = 0

        while i < n - 1 and arr[i] < arr[i + 1]:
            i += 1

        if i == 0 or i == n - 1:
            return False

        while i < n - 1 and arr[i] > arr[i + 1]:
            i += 1
        return i == n - 1

```