Tree Walker:
A
  A.1
  A.2
  A.3
B
  B.1
    B.1.1
    B.1.2
  B.2
    B.2.1
C
  C.1
    C.1.1
      C.1.1.1
      C.1.1.2
      C.1.1.3
    C.1.2
      C.2.1
D

Tree Walker:
D
      C.2.1
    C.1.2
      C.1.1.3
      C.1.1.2
      C.1.1.1
    C.1.1
  C.1
C
    B.2.1
  B.2
    B.1.2
    B.1.1
  B.1
B
  A.3
  A.2
  A.1
A