LESSON 6 · PATTERN GUIDE

Hashing / Frequency Maps

Use a set or map to track what has appeared and how often, usually with constant-time average lookup.

FIRST, LEARN THE PATTERNA hash set answers whether a value was seen; a frequency map stores counts or associates values with useful metadata.

The task asks for duplicates, complements, counts, anagrams, or quick membership checks.

How to approach it

Choose the key that identifies equivalent items, update its count as you scan, and avoid repeated nested searches.

Watch out for

Hash maps use extra memory and do not preserve sorted order unless you choose an ordered structure.

A CLOSER EXPLANATION

A hash map turns repeated searching into direct lookup by a key. Pick a key that represents what should be considered equal: a number itself, a normalized word, or a character. Frequency counts can answer both membership and multiplicity questions in one pass.

NOW YOU TRY

Practice question

Do the words 'listen' and 'silent' contain the same letters with the same frequencies?

EXAMPLEfirst = 'listen', second = 'silent'Give your answer before revealing the explanation.

Show a hint

Count character frequencies rather than comparing character positions.

Show an answer

Yes. Each letter occurs once in both words.

CONNECT IT TO REAL LIFE

Hashing / Frequency Maps at work

A ticket system receives IDs [A12, B04, A12, C31, B04, A12]. Which ticket ID appears most often, and how many times?

Show one possible approach

A12 appears 3 times. Increment a map entry for each ID as the stream is read, then track the largest count.

Practice next

Use these problem IDs to find additional practice for this pattern:

149128217242347