def at(i)
      return nil if @map.length == 0
      if i == 0
        left = 0
      else
        left = @map[i-1][0]+1
      end

      [@map[i][1],left,@map[i][0]]
    end
def at(i)
      return nil if @map.length == 0
      if i == 0
        left = 0
      else
        left = @map[i-1][0]+1
      end

      [@map[i][1],left,@map[i][0]]
    end
For the region with index i, return an array of the form [value, left, right]