Valid Mountain Array

Problem

Return True if arr is a mountain array: strictly increasing to a peak and then strictly decreasing, with at least three elements.

Walk up while values increase, require that the peak is neither the first nor the last element, then walk down and require reaching the end. Equal neighbours invalidate the mountain.

Hint

The up-walk must stop before the last element and the down-walk must reach it.

Test cases

case 1

in: [[2,1]]

expected: false

case 2

in: [[3,5,5]]

expected: false

case 3

in: [[0,3,2,1]]

expected: true

case 4

in: [[0,1,2,3,4,5,6,7,8,9]]

expected: false

case 5

in: [[1,3,2]]

expected: true

case 6

in: [[9,8,7,6,5]]

expected: false

Starter codePython

def valid_mountain_array(arr):
    # Your code here
    pass

6 test cases · run your code against them in the editor