import random
n=9
b=[[1,1],[2,2],[3,3],[4,4],[5,5],[6,6],[7,7],[8,8],[9,-1]]
head=0
a=[]
for i in range(6):
k=random.randint(0,n-i-1)
if k==0:
a.append(b[head][0])
①
else:
p=head
for j in range(k):
pre=p
p=b[p][1]
a.append(b[p][0])
②
划线处应填入的代码为( )