TalentBuddy Plane Tickets - too slow!

plane_tickets.py

from bisect import bisect_left

def index(a, x):
    'Locate the leftmost value exactly equal to x'
    i = bisect_left(a, x)
    if i != len(a) and a[i] == x:
        return i
    raise ValueError

def get_journey(departure_ids, destination_ids):
    departure_ids.sort()
    destination_ids.sort()
    
    departure_set = set(departure_ids)
    destination_set = set(destination_ids)
    
    diff = list(departure_set.symmetric_difference(destination_set))
    
    if diff[0] in departure_set:
        origin = diff[0]
        last = diff[1]
    else:
        origin = diff[1]
        last = diff[0]

    place = origin
    itinerary = str(origin)
    
    ticket = index(departure_ids, place)
    while place != last:      
        destination = destination_ids[ticket]
        itinerary = itinerary + " " + str(destination)
        
        ticket = index(departure_ids, place)
        place = departure_ids[ticket]
        
    print itinerary