Hashing Patterns (capstone)

The hash map is a swiss-army knife

You met the hash map in lesson one. This capstone is about the three patterns it unlocks — the ones that solve a huge fraction of string and array problems:

• Frequency counts — how many times each thing appears.
• Grouping by a canonical key — bucket items that share some computed key.
• Seen-sets — remember what you've encountered (Two Sum was this).

Recognizing which pattern a problem wants is most of the battle.