Solution to the cycling problem

races.py

list_of_races = [[('c','d'), ('a','b'),('b','c')], 
                 [('C', 'B'), ('D', 'C'), ('B', 'E'), ('A', 'D')]]

for races in list_of_races:
    print(races)
    races = dict(races)
    start_city = (set(races.keys()) - set(races.values())).pop()
    end_city  = (set(races.values()) - set(races.keys())).pop()
    all_cities = list(set(races.keys()).union(set(races.values())))

    order = start_city
    while all_cities and start_city!= end_city:
        next_city = races[start_city]
        order += next_city
        all_cities.remove(next_city)
        start_city = next_city

    print(order)

Problem
A cycling race is composed of many legs. Each leg goes from one city(A) to another(B). Cyclists then rest for the night and start the next leg of journey from (B) to (C). If the legs are represented by Tuples (A,B), (B,C), (C,D)...and given a list of tuples out of order example [(C,D),(A,B),(B,C)...] can you print out the correct order of cities in the race (example "A B C D..")

Example : [(A C) (B D) (C B)]

output : A C B D

Example : [(C B) (D C) (B E) (A D)]

output : A D C B E