foundation

Arrays & Hashing

Use arrays, hash maps, and sets for fast lookup, counting, grouping, and deduplication.

Problems

Problems in Arrays & Hashing

Practice problems that share this primary solution pattern and compare the clues that reveal it.

A dark-themed chat interface displaying an AI assistant conversation starter on a screen.

beginner

7 min read

Two Sum

A strong Two Sum solution replaces repeated pair scanning with one sharper question: has the array already shown us the value this number needs?

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A modern open laptop with a black screen placed on lush green grass, symbolizing technology and nature.

intermediate

8 min read

Valid Sudoku

A Sudoku validator does not solve the puzzle. It tracks whether the digits already placed violate any row, column, or 3×3 box constraint.

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Person interacts with robot images on a screen in a dark room, highlighting technology use.

intermediate

9 min read

Group Anagrams

A useful Group Anagrams solution does not compare every string with every existing group. It assigns each string a stable identity based on its character…

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