# 941. Valid Mountain Array

#### Easy

***

Given an array of integers `arr`, return *`true` if and only if it is a valid mountain array*.

Recall that arr is a mountain array if and only if:

* `arr.length >= 3`
* There exists some `i` with `0 < i < arr.length - 1` such that:
  * `arr[0] < arr[1] < ... < arr[i - 1] < arr[i]`
  * `arr[i] > arr[i + 1] > ... > arr[arr.length - 1]`

![](https://assets.leetcode.com/uploads/2019/10/20/hint_valid_mountain_array.png)

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**Example 1:**

```
Input: arr = [2,1]
Output: false
```

**Example 2:**

```
Input: arr = [3,5,5]
Output: false
```

**Example 3:**

```
Input: arr = [0,3,2,1]
Output: true
```

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**Constraints:**

* `1 <= arr.length <= 104`
* `0 <= arr[i] <= 104`

```python
class Solution:
    def validMountainArray(self, arr: List[int]) -> bool:
        if len(arr) < 3:
            return False
        left = 0
        while left < len(arr)-1 and arr[left] < arr[left+1]:
            left += 1
        right = len(arr)- 1
        while right >= 1 and arr[right] < arr[right-1]:
            if right < left:
                return False
            right -= 1
        if left == right == 0 or (left == right == len(arr)-1):
            return False
        return left == right
```