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📜 WAYPOINT LESSON

Diagnose before code

⭐⭐ intermediate⏳ 12 min read📍 Lesson 125 of 256

Return type → data → composition: naming the machines from the contract.

No module label this time. Each problem below mixes several machines; your first job is diagnosis, not coding. The two-minute ritual:

  1. Read the contract; name the return type. A count → counter machine. A String → builder machine. A boolean → flag/existence. A position → sentinel. A new collection → accumulator.
  2. Name the data. Array vs ArrayList decides the loop family and the size API. String decides charAt/substring.
  3. Name the composition. Does the answer need a pass over data AND a pass over positions (columns)? A filter inside a scan? A max over computed per-row values?
  4. Only now write code — the machines assemble themselves once named.

Example diagnosis: "Return the number of distinct values in the strictly-positive half of the grid's first row." → return type: count (counter). Data: array row. Composition: filter (positive) + dedup (backward-looking pairwise). The code is three machines you already own — the reading is the work.