from collections import deque

o = [[1,2],[3,4],[2,3],[5,4]]

def group_lists(list_of_lists):
  '''Combine all items in list_of_lists that share
  an element recursively, until no two share an item'''
  queue = deque( sorted(list_of_lists, key=lambda x:min(x)) )
  grouped = []
  while len(queue) >= 2:
    print('queue length', len(queue))
    s1 = set(queue.popleft())
    s2 = set(queue.popleft())
    if s1 & s2:
      queue.appendleft(s1 | s2)
    else:
        grouped.append(s1)
        queue.appendleft(s2)
  if queue:
    grouped.append(queue.pop())
  return grouped

group_lists(o)