Line Consistency

Medium Used by Smart Hint

Every way to fill this row or column's remaining heights that matches its clue was checked — a height eliminated here doesn't appear in any of them.

Cells that establish the pattern
Where the conclusion lands
Height ruled out

How it works

The 2 on the left says exactly two buildings show along this row, and the 4 is already placed at the far end.
Try every way to finish the row: if the first cell were 1 or 2, some taller building before the 4 would poke out and make three visible.
Only a 3 first — hiding the 1 and 2 behind it, then the 4 — shows exactly two. The first cell is 3.