Arrays

Problem	Complexity	Key Pattern
Group Anagrams	O(n * k log k) where n = number of strings, k = max string length	Grouping items by equivalence class — anagrams, isomorphic strings, etc.
Product Except Self	O(n)	Prefix/suffix accumulation without division — product, sum, or any
Top K Frequent	O(n)	Frequency counting + selection — "top K", "most common", "least common".
Two Sum	O(n)	Any problem asking "find pair with property X" — hash map for O(1) lookups.