📜 WAYPOINT LESSON
Checkpoint — The Lost Boundary
⭐⭐⭐ advanced⏳ 25 min read📍 Lesson 239 of 281
Lower-bound binary search: the shipped code loses the answer when it is the last index (closed-interval + early hi shrink). Fix it; the tests pin the boundary and the all-tie case.
Checkpoint — The Lost Boundary. The provided binary search looks
canonical but answers the wrong predicate: a[mid] <= x makes it return
the first index strictly GREATER than x (upper_bound), so when x equals
qualifying elements — the last-element boundary, or an all-equal array —
it reports −1. The repair is one character: < instead of <=, with the
post-loop existence check intact. The boundary and all-tie tests are
exactly the shapes the stumble cannot survive.