Valid Mountain Array
   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] < A[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:            class Solution {          public boolean validMountainArray(int[] arr) {              int n = arr.length, i = 0, j = n - 1;                     while (i+1  < n &...