def traceback_array(r = @max_row, c = @max_col)
      trace = []
      traceback(r,c) do |i,j,flags|
        trace << [i,j,flags]
      end
      trace
    end
def traceback_array(r = @max_row, c = @max_col)
      trace = []
      traceback(r,c) do |i,j,flags|
        trace << [i,j,flags]
      end
      trace
    end
traceback
 Like traceback, but returns an array of the traceback instead of
 yielding blocks.
 @param [Integer] r The row to traceback from
 @param [Integer] c The column to traceback from