Browse 14 Hash Table Coding Challenge
Hash table problems are the ultimate trade-off: use a little memory to turn slow O(n²) logic into sharp, fast O(n) coding solutions.
Problems (14 total)
About Hash Table Problems
If there's one data structure you should be best friends with before any coding interview or specific coding challenge, it's the hash table. You've probably used it already in other problems — a Python dict or a JavaScript Map.
The reason hash table problems come up so often is that they unlock a simple trade-off: spend a little extra memory, and suddenly a nested-loop O(n²) solution collapses into a clean O(n) pass. That's the deal. You iterate once, store what you've seen in the map, and on the next element you just check if the answer is already sitting there waiting for you.
Most problems in this category follow the same few moves. You're either counting how often something shows up, looking up whether a complement exists (classic Two Sum), grouping items that share a common key (like anagrams sharing a sorted form), or using prefix sums as map keys to find subarrays in one shot. Once you recognise the pattern, the code almost writes itself.
Budibadu gives you hash table problems that train exactly this instinct: spot the key, store the right value, and turn messy brute force into something sharp and fast. If you want to get comfortable with Two Sum patterns, frequency maps, and duplicate checks, challenge yourself here and see how quickly your coding levels up.