add Ternary Search code[C++]

[Yak0xff]

Fixes #481

By submitting this pull request I confirm I've read and complied with the below declarations.

• [x] I have read the Contribution guidelines and I am confident that my PR reflects them.
• [x] I have followed the coding guidelines for this project.
• [x] My code follows the skeleton code structure.
• [x] This pull request has a descriptive title. For example, Added {Algorithm/DS name} [{Language}], not Update README.md or Added new code.
• [x] This pull request will be closed if I fail to update it even once in a continuous time span of 7 days.

[singhpratyush]

Hi! Thanks for showing interest in contributing to this repository.

Please make sure that you have ticked the points mentioned in PR description correctly.

Thanks again!

[singhpratyush]

@RobinChao: Are you still on it?