Mountain Array

ARRAY

Problem

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 index i (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]

Examples

isMountainArray([1,4,7,9,8,5,3])
// returns true
// The array forms a mountain with a peak at arr[3]

isMountainArray([5,8,12,12,9,7])
// returns false
// The array fails to form a mountain because
// arr[2] == arr[3], indicating a flat segment

isMountainArray([5,6,5,4,3])
// returns true
// The array forms a mountain with a peak at arr[1]

ARR