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 arr[i + 1] > ... > arr[arr.length - 1] Example 1: Input: arr = [2,1] Output: false Example 2: Input: arr = [3,5,5] Output: false ..