Group by Canonical Key: When to Use It

Start with the idea

A problem is a good match for Group by Canonical Key when it asks for anagram groups, bucketing records, equivalence classes. Here the concrete task is: Group words by sorted letters and return the size of the largest group. The visible input [eat,tea,tan,ate,nat] is an example, not a definition of the whole problem. Ask what operations the solution must support and what the input guarantees. Look for repeated membership, grouping, or a reusable key representation. A sorted structure may be preferable when deterministic order is part of the requirement. The right choice follows from those guarantees, not from a familiar noun in the prompt.

The decisive invariant is: Transform each item to a stable key and collect items sharing that key. Try to say why it applies to the task without quoting the implementation. Which fact becomes known after one update, and which possible answers remain? A key-derived representation lets later work reuse an earlier fact. It may be a set, a map from keys to state, compressed ranks, or a rolling fingerprint; name which one this pattern uses. If that claim cannot be maintained on a legal input, the pattern is wrong or one of its preconditions is missing. A counterexample is more useful than a vague warning: change one property of the sample, trace the first two steps, and identify the exact claim that fails.

Compare costs before committing. The advertised bound is O(total input) expected time · O(total input) space; identify the operation responsible for it and the extra memory it needs.