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941. Valid Mountain Array

Difficulty Topics

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

valid-mountain-array.py
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