Sudoku puzzle is not good task to measure skill of automation tester.
I would like to suggest some good task which you can assign to test skill of selenium automation tester :
1 - Basic test for register form where all type of fields available , ie : http://www.toolsqa.com/automation-practice-form/
2 - Data driven task , For ex: Get data from excel , Import data to excel
3 - Implement explicit wait , work with Iframe , child windows
4 - Integrate TestNG and generate test report
5 - Alerts handling
6 - OOPs concepts implementation
7 - Work with site when element changes dynamically
Above are my suggestions , Everything is depends on your need , your projects , your environment. Make sure whenever you are going to hire any candidate for automation testing, you should ask question for skills which you are using in your daily job or your QA team is using.
There is no meaning to ask questions about Sudoku puzzle because once candidate joins he/she will not work to create Sudoku puzzle. It is good idea to measure logical skill but give more importance to actual need.