In Codeforces Round #674 (Div. 3) many solutions for problem C got Hacked. After looking into it I found these solutions show TLE for the hack_test_case. After the hacking phase when I looked into hack_test_case and I found it to be same as tc in sys test.

Soln hacked : link

(same code) AC : link

Doubts:

1. O(1000*(sqrt(1e9)) = approx(31622776) can get TLE?

2. How can a solution get accepted and hacked on the same test case?

There are many soln that got hacked on the same tc and pass the same tc when submitted again.

1 2 and many more...

( UPD : I found the above code got AC for GNU C++17 (64) but got TLE for lower version.Can we say GNU C++17 (64) compiler is better optimized? should one always use it? ).