ArrayList mutation discipline

ArrayList mutation discipline. Three legal strategies, one forbidden one:

• Reverse index loop (for (int i = list.size() - 1; i >= 0; i--)): removals cannot affect not-yet-visited indexes. The safest remove-matches loop.
• Build a new list: read the old, append survivors. Costs memory, never misindexes.
• Write-index over the same list via set — advanced, rarely needed on the exam.

Forbidden: removing from an ArrayList inside a for-each loop — ConcurrentModificationException. (The AP subset does test this as a "why does this crash" MCQ.)

Two-cursor merge (interleave): advance whichever input still has elements, alternating. The drain phase — one list already exhausted — is the boundary the distractors encode: they drop the tail of the longer list.