When Hashing Fails

In competitive programming, some judges create adversarial test cases that cause many hash collisions, degrading $O(1)$ to $O(n)$. Signs of a hash attack: TLE on unordered_map but AC on map. Solutions:

$1.$ Use tree-based structures (map in C++, TreeMap in Java). $O(\log n)$ is slower but guaranteed.

$2.$ Use a custom hash function that's harder to attack.

$3.$ In Python, dict is generally resistant but not immune.