941. Valid Mountain Array

Easy

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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

class Solution:
    def validMountainArray(self, arr: List[int]) -> bool:
        if len(arr) < 3:
            return False
        left = 0
        while left < len(arr)-1 and arr[left] < arr[left+1]:
            left += 1
        right = len(arr)- 1
        while right >= 1 and arr[right] < arr[right-1]:
            if right < left:
                return False
            right -= 1
        if left == right == 0 or (left == right == len(arr)-1):
            return False
        return left == right