树的遍历

任务总结
root
a1
b1
c1
c2
c3
b2
c4
c5
b3
c6
c7
b4
c8
c9
a2
b5
c10
b6
c11
c12
b7
c13
c14
c15
c16
a3
b8
c17
b9