Description
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
with0 < 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
Solution
Python3
class Solution:
def validMountainArray(self, arr: List[int]) -> bool:
n = len(arr)
if n < 3: return False
i, j = 0, n - 1
while i + 1 < n and arr[i] < arr[i + 1]:
i += 1
while j > 0 and arr[j] < arr[j - 1]:
j -= 1
return i == j and j != n - 1 and i != 0