Getting WA on Subtask 1 Task# 0 what does it mean

what does this mean. rest all test cases are passing

This means your code is not producing same output as desired according to constraints given in subtask 1

yeah i get that but all other test cases in subtask 1 are passing. So is task# 0 purposely named so or other reason?

Every problem has some corner cases where the general solution fails you have to figure out which are those cases where your logic could fail try debugging using various other cases

No special labeling here. Your solution just is not 100% correct. Could just be one test case or many of them. Just try to figure out where the code is failing. Apart from that, there is no way for us to know what test case is failing.