941. Valid Mountain Array-有效的山脉数组

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 <= 10^4
• 0 <= arr[i] <= 10^4

Solution in python:

class Solution:
    def validMountainArray(self, arr: List[int]) -> bool:
        i = 0
        j = len(arr)-1
        if j < 2 or arr[1] < arr[0] or arr[-1] > arr[-2]:
            return False 
        while i < len(arr):
            if arr[i+1] > arr[i]:
                i += 1
            else: break
        while j > 0:
            if arr[j-1] > arr[j]:
                j -= 1
            else: break
        return i == j