unordered_map offers average O(1) time for insert, find, and erase ,it can still cause Time Limit Exceeded (TLE)
1. Worst-Case Complexity is O(n)
Hash collisions can degrade performance from O(1) to O(n), especially with poor key distribution.
Use map (O(log n)) for consistent performance.
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Thats all because of hash collision, all numbers going to one bucket. But if you want to use unordered_set or unordered_map, you should have a custom hash function, which you get from GPT, but not in contest. You can do it out of contest, like write your template.