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The “Many random cases” validator when the solution is submitted fails.
The test cases do not cover time execution issues. If your algorithms are not efficient they will not pass the validators. The test cases must add a long execution test case that will fail for not optimal algorithms.
The solution passed the provided test cases and also some customized test cases with very big prime numbers (to exclude this as a reason as was not covered in the test cases). The solution is general, so, I had no idea what case is not covered. Part of creating a puzzle is to be sure that the test cases are well selected and validators are double checked against correct answers and corresponding test cases. Not offering any indication why a validators fail makes the code difficult to debug. I guess the efficiency was an isssue and improved the speed, which worked.