Hashtable A hashtable (hash map, dictionary, or hash set) maps keys to values using a hash function for O(1) average lookups. Complexity Operation Average Lookup / Insert / Delete O(1) Iteration O(n) Collision Handling Chaining: Store colliding entries in a linked list at the same index. Open Addressing: Probe for the next available slot. Python Usage dict (hash map) d = {"apple": 5, "banana": 3} d["orange"] = 8 print(d["apple"], d.get("pear", 0)) del d["banana"] "orange" in d # membership for k, v in d.items(): # iterate d.clear() set (hash set) s = set() s.add(10); s.add(20) 10 in s # True s.remove(10) # raises KeyError if missing s.discard(50) # safe remove len(s); s.clear() Related LeetCode Questions # Problem Difficulty Technique 1 Two Sum Easy Hash map for complement lookup 49 Group Anagrams Medium Hash map keyed by sorted string 128 Longest Consecutive Sequence Medium Hash set for O(1) neighbor search 217 Contains Duplicate Easy Hash set for membership test