Let's say I have a list of tuples like:

x = [(2, 18), (18, 5), (3, 2)]

How can I sort this list of tuples based on the unique occurrence of the values in the tuples?

For example, since the number 3 only occurs in the tuple (3, 2) and is the first value of the tuple, it should be the first entry in the list. This entry is followed by (2, 18) because the second value (2) of (3, 2) occurs in the first value of (2, 18). And finally, the last entry in the list should be (18, 5), since its first value matches the last value of (2, 18).

Expected result:

[(3, 2), (2, 18), (18, 5)]

Pls tell me if you need more info.

CodePudding user response：

Use a recursive function to pick dominos one by one:

def get_elements_filtered_by_first_values(original_list, first_value):
   return [each_element for each_element in original_list if each_element[0] == first_value]

def add_next_domino(list_of_remained_dominos, list_of_picked_dominos):
   possible_domino_list_to_pick = get_elements_filtered_by_first_values(list_of_remained_dominos, list_of_picked_dominos[-1][1])
   if not len(possible_domino_list_to_pick):
      return None
   for each_possible_domino_to_pick in possible_domino_list_to_pick:
      new_list_of_picked_dominos = list_of_picked_dominos   [each_possible_domino_to_pick]
      new_list_of_remained_dominos = list_of_remained_dominos[:]
      new_list_of_remained_dominos.remove(each_possible_domino_to_pick)
      if not len(new_list_of_remained_dominos):
         return new_list_of_picked_dominos
      pick_domino_result = add_next_domino(new_list_of_remained_dominos, new_list_of_picked_dominos)
      if pick_domino_result is not None:
         return pick_domino_result
   return None

x = [(2, 18), (18, 5), (3, 2)]
eligible_elements = [each_element for each_element in x if each_element[0] not in map(lambda x: x[1], x)]
while len(eligible_elements):
   next_eligible_element = eligible_elements.pop()
   return_list = add_next_domino([each_element for each_element in x if each_element != next_eligible_element] ,[next_eligible_element])
   if return_list is not None:
      print(return_list)
      break

The output:

[(3, 2), (2, 18), (18, 5)]