Valid Mountain Array

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

Solution

class Solution:
    def validMountainArray(self, arr: List[int]) -> bool:
        i = 0
        j = len(arr) - 1
        n = len(arr)

        while i + 1 < n and arr[i] < arr[i + 1]:
            i += 1
        while j > 0 and arr[j-1] > arr[j]:
            j -= 1
        return 0 < i == j < n - 1