Algorithm Atlas

Preparing your learning space

Loading your lessons and saved progress.

Learn HashMap with visual DSA lessons

Learn HashMap with 2 DSA patterns and 5 visual lessons. Follow step-by-step animations and synced C#, Python, and Java code.

A HashMap stores a value under a key for fast lookup.

Frequency Map / Counting

Count how often each value appears and use those counts to decide.

Majority Element Easy · Key-value memory
Top K Frequent Elements Medium · Key-value memory
Sort Characters By Frequency Medium · Key-value memory
Task Scheduler (frequency-based greedy) Medium · Key-value memory

Prefix-Sum with Map

Store earlier running totals so matching subarrays can be counted instantly.

Count Subarrays with Sum K Medium · Key-value memory