I try not to be too hard on Jeffries in this case. Although he failed to write a Sudoku solver, expressing the requirements in the form of tests can be a decent way to start in cases where you know some valid outputs for a series of given inputs.
I tend to solve Leetcode/Hacker Rank problems this way, and it works pretty well. The catch is that the tests won't tell you what algorithm will best solve the problem. And there's of course no guarantee the tests you've thought up will cover all edge cases.
