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]`

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

Constraints:

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