Last contest, the solution of problem E has a complexity of $$$O(n2^m)$$$, which costs about $$$10^8$$$ calculations. On many programming platforms, that is really risky.
For example, in a chinese contest 'THUWC', a solution with a bottleneck at unordered_map and a data size of $$$5\times 10^5$$$ can lead to TLE (the time limit is 1s, and the language is C++17).
Then how many plus/minus operations can Codeforces' server perform in a second?
lol this solution is $$$\mathcal{O}(n \cdot 2^m \cdot m)$$$ and it still (barely) passed.
Btw, your chinese contest might just have an unordered_map hash hack. It's probably not related to the judging server.