• Problem: XXOR

• Submission-1: Link (Runtime Error)

• Submission-2: Link (Accepted)

• The difference between both the submissions is just the two cout<<""; statements, in the two "n" range for loops.

• The cout<<""; statements are uncommented in Submission-2 and commented in Submission-1. And guess what, these statements convert a RE Submission to an AC Submission.

• The code in Submission-1, for a Valid Random Input, runs finely in Codeforces' Custom Test, whereas it gives RE in AtCoder's Custom Invocation. (You may try, for more clarification and to find the reason!)

• I found it weird! I am totally clueless about why this is happening. Can anyone please justify or explain, why is this happening?

• Thank you!

UPD: It seems like, the cout<<""; statement(s) in the "n" range for loops, avoid (stop) the optimisation(s), which -OFast + "avx2" try to make, hence, converting the RE solution into an AC solution.