Valid Mountain Array

valid_mountain_array.py

"""
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] < A[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
"""
class Solution:
    def validMountainArray(self, arr: List[int]) -> bool:
        n = len(arr)
        i = 0
        # walk up
        while i < (n - 1) and arr[i] < arr[i + 1]:
            i += 1
        if i == 0 or i == (n - 1):
            return False
        # walk down
        while i < (n - 1) and arr[i] > arr[i + 1]:
            i += 1
        return i == (n - 1)