The code looks good and well-organized. 
Here are a few suggestions:
- Use `const` wherever possible to make the code more readable and safe. For example, in `checkWinner()` and `isUserInputValid()`, the function parameters can be marked `const`.
- Use range-based `for` loop wherever possible to make the code more readable and safe. For example, in `isUserInputValid()`, the `for` loop can be replaced with a range-based `for` loop.
- Use enum instead of `std::vector<std::string>` for game options. This makes the code more readable and safer.
- Use auto wherever possible to make the code more readable and safer. For example, in `checkWinner()`, the return type can be replaced with `auto`.
- Avoid using using `namespace std` as it is considered bad practice.
- Add error handling to the code, for example, in case of invalid user input.