Pebbling a checkerboard. We are given a checkerboard which has 4 rows and n d has an integer Cli,j] written in each square, wherei E [4] and j E [n We are also given a set of 2n pebbles, and we want to place some or all of these on the checkerboard (each pebble can be placed on exactly one square) so as to maximize the sum of the integers in the squares that are covered by pebbles. There is one constraint for a placement of pebbles to be legal, no two of them can be on horizontally or vertically columns. an adjacent squares (diagonal adjacency is fine) (a) Determine the number of legal patterns that can occur in any column (in isola- tion, ignoring the pebbles in adjacent columns) and describe these patterns. (You can draw pictures.) Call two patterns compatible if they can be placed on adjacent columns to form a legal placement. Let us consider subproblems consisting of the first k columns 1