Just now my friend(parallelist) found a wrong solution(83183287) of an old problem. He tried to hack it but couldn't find the hack button. I guess that it's because he seldom participates in contests so he ask me to do it and gave me his hack tests. But there is NO hack button either on my screen. "Maybe I should solve this problem first?" I think. Then I do a copy paste to get AC. Well, there's still no hack button.
I am sorry for my friend, wondering if it is true that on Codeforces, you can not make a hack on an old problem? Thanks in advance.
As written in https://codeforces.net/blog/entry/68204, you can only uphack for 7 days after the contest ends.
Oh, thank you! I didn't know the hack system before.