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📜 WAYPOINT LESSON

The write-index pattern

⭐⭐⭐ advanced⏳ 10 min read📍 Lesson 182 of 256

In-place compaction with a write cursor: dedup, remove-matches, move-zeros — one pattern, every compact FRQ.

The write-index pattern (in-place compaction):

int w = 0;                       // next slot to write
for (int i = 0; i < a.length; i++) {
    if (keep(a[i])) { a[w] = a[i]; w++; }
}
// w = new logical length; cells a[w..] are garbage-but-safe

This solves dedup-on-sorted, remove-matches, move-zeros — every "compact an array" FRQ the exam has ever written. The pattern's discipline: w never looks back, i never skips. W solutions in this module fail exactly one boundary (the last element or the first) so you can see which boundary your own code forgot.

Rotation by k: copy to temp with tmp[(i + k) % n] = a[i], then copy back. Off-by-one candidates: starting the modulo at 1, or rotating the wrong direction. Check with n=1 (no change) and k=n (no change) — both must be no-ops.