gistfile1.txt

[avagin@laptop ~]$ cat t.py 
import sys

items = {}

class Item:
    def __init__(self, key, parent):
        self.parent = parent
        self.key = key
        self.children = []
    pass

f = open(sys.argv[1])
k = f.readline().split()[0]
root = item = Item(k, None)
items[k] = item
for l in f:
    k, p = l.split()
    item = Item(k, items[p])
    items[k] = item
    items[p].children.append(item)

next_level = [root]

r = 0;

while len(next_level):
    level = next_level
    next_level = []
    r = r + 1
    level.reverse()
    for i in level:
        next_level.extend(i.children)
        print i.key,
    print

[avagin@laptop ~]$ cat input 
F
B F
G F
A B
D B
I G
C A
E A
H I
[avagin@laptop ~]$ python t.py input 
F
G B
D A I
H E C