941-valid-mountain-array

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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 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

Solution(Python)

class Solution:
    def validMountainArray(self, arr: List[int]) -> bool:
        n = len(arr)
        if n < 3:
            return False
        i = 0

        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