I have no idea how to solve this task.

I am thinking if I give up this task... Would you give me some advice?

I got "MemoryError" with this code, although I know I use "for" too much.

import collections def check_connection(network, first, second): box=collections.defaultdict(list) result={} for l in network: (x,y) = l.split('-') box[x].append(y) box[y].append(x) for k1, v1 in box.iteritems(): for k2, v2 in box.iteritems(): if k1 != k2: for x in v2: if k1 in v2 or x in v1: v1.extend(v2) result[k1] = set(v1) for key in result.keys(): if first in result[second]: return True else: return False