Tree Walker: 
A
  A.1
  A.2
  A.3
B
  B.2
    B.2.1
C
  C.1
    C.1.2
      C.2.1
D

Tree Walker: 
D
      C.2.1
    C.1.2
  C.1
C
    B.2.1
  B.2
B
  A.3
  A.2
  A.1
A