Value-to-Index Map: When to Use It

Start with the idea

A problem is a good match for Value-to-Index Map when it asks for distance between repeats, sparse lookup, reconciliation. Here the concrete task is: Return the shortest distance between equal values. The visible input [5, 1, 9, 5, 1] 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: Remember the latest or earliest position associated with each value. 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(n) expected time · O(k) space; identify the operation responsible for it and the extra memory it needs.