C++ code for Mobile Numeric Keypad

I am LGM-SOC participant and want to add the C++ code for Mobile Numeric Keypad.

Description: Given the mobile numeric keypad. You can only press buttons that are up, left, right, or down to the current button. You are not allowed to press bottom row corner buttons (i.e. * and # ). Given a number N, the task is to find out the number of possible numbers of the given length.

### Programming language

- [ ] C
- [ ] C++
- [ ] Java
- [ ] Python

Start by locating the repository's C++ solution area and existing conventions for coding problems; the issue does not name a file or test. Implement the Mobile Numeric Keypad count for a given length, excluding * and # and allowing only adjacent vertical or horizontal moves, then verify the expected counts for representative lengths.

Written by the indexing model from the issue text.