1 [M],[M]
   2 [],[]
   3 [M],[M]
   4 [],[]
   5 null
   6 [M],[M]
   7 null
   8 [M],[M]
   9 null
  10 null
  11 M
  12 null
  13 ^
  14 [M],[M]
  15 null
  16 [M],[M]
  17 null
  18 null
  19 M
  20 null
  21 ^