added c++ code for 845. Longest Mountain in Array

[IliasKhan]

Pull Request Template

Description

Let's call any (contiguous) subarray B (of A) a mountain if the following properties hold:B.length >= 3
There exists some 0 < i < B.length - 1 such that B[0] < B[1] < ... B[i-1] < B[i] > B[i+1] > ... > B[B.length - 1]
(Note that B could be any subarray of A, including the entire array A.)
Given an array A of integers, return the length of the longest mountain. Return 0 if there is no mountain.

CodeExplain:
while treversing the array..we have to just count increasing and decreasing numbers.
and maximize the addition of increasing and decreasing numbers.

Put check marks:

Have you made changes in README file ?

• Added problem & solution under correct topic.
• Specified Space & Time complexity.
• Specified difficulty level, tag & Note(if any).

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Please also note any relevant details for your test configuration.

• input1: [2,1,4,7,3,2,5]
• input2: [2,2,2]

Make sure all below guidelines are followed else PR will get Reject:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code so that it is easy to understand
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

[GouravRusiya30]

Very nicely documented @IliasKhan