Valid Mountain Array

'''
Checks:
1. The whole array can neither be strictly increasing or strictly decreasing entirely.
2. The strictly decreasing part should come after the strictly increasing part and together they should make up the entire array
'''
#1. One Pointer
class Solution:
    def validMountainArray(self, arr: List[int]) -> bool:
        i, n = 0, len(arr)
        while i<n-1 and arr[i]<arr[i+1]:
            i += 1
        if i==0 or i==n-1:
            return False
        while i<n-1 and arr[i]>arr[i+1]:
            i += 1
        return i==n-1

#2. Two pointer
#credits to lee215 solution for idea
class Solution:
    def validMountainArray(self, arr: List[int]) -> bool:
        n = len(arr)
        i, j = 0, n-1
        while i<n-1 and arr[i]<arr[i+1]:
            i += 1
        while j>0 and arr[j]<arr[j-1]:
            j -= 1
        return i == j and 0 < i < n-1