```# Same logic as my first solution, simply make it more OO and clean up a bit.

class PenAndPaperGame
MOVES = [[3,0], [-3,0], [0,3], [0,-3], [2,2], [2,-2], [-2,2], [-2,-2]]

def initialize(size)
@size = size
@largest = @size * @size
@board = Array.new(@size) { Array.new(@size) }
end

def to_s
return "No solution." unless @board[0][0]
digits = @largest.to_s.size
" #{'-' * (digits+2) * @size}\n|" +
(@board.map { |l| l.map{|e| " %#{digits}d " % e}.join }.join("|\n|")) +
"|\n #{'-' * (digits+2) * @size}\n"
end

def solve
@size.times { |x| @size.times { |y| return self if jump(1, x, y) } }
self
end

private

def valid(x, y)
x >= 0 && x < @size && y >= 0 && y < @size && !@board[x][y]
end

def nb_exit(x, y)
MOVES.inject(0) { |m, (i, j)| valid(x+i, y+j) ? m+1 : m }
end

def jump(nb, x, y)
@board[x][y] = nb
return true if nb == @largest
MOVES.map { |i,j| [x+i, y+j] }.select { |i,j| valid(i,j) }.
sort_by { |i,j| nb_exit(i,j) }.each { |i,j| return true if
jump(nb+1, i, j) }
@board[x][y] = nil
end
end

if __FILE__ == \$0
size = (ARGV[0] || 5).to_i
puts PenAndPaperGame.new(size).solve
end

```