Around the edges have been placed letters indicating the closest non-blank entry in that row or column. For example, the A to the right of row 2 indicates that the last non-blank in that row should be an A. The first non-blank in that row will be a D as indicated by the D to the left of that row.
A D
++===+===+===+===+===++
|| | | | | ||
++---+---+---+---+---++
D || | | | | || A
++---+---+---+---+---++
|| | | | | || B
++---+---+---+---+---++
|| | | | | ||
++---+---+---+---+---++
B || | | | | || A
++===+===+===+===+===++
C
From Mensa Puzzle Calendar 2019 by Fraser Simpson, Workman Publishing, New York. Puzzle for February 19.
