0: 0
1:
2:
3:
4:
5:
6: 6
7: 7
8: 8 9 11 3 4 5 15
9:
10: 10 1 2
11:
12: 12
13: 13
14: 14
15:
16: 16
17: 17 18 19 20
18:
19:
20:
