Hash maps and anti-hash tests

unordered_map is a hash map with average O(1) operations, often faster than map. On Codeforces, though, other contestants can hack your solution: submit a test designed to break it. With the default hash function, a test can make every key land in the same bucket, and each operation becomes O(n), turning an accepted solution into time limit exceeded after the round.

A custom hash with a random seed, such as the splitmix64 function, defends against this, and so does simply using map. Know both options and when the extra speed is worth it.