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::vectorstd::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.

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.