Enigmatic Code
Programming Enigma Puzzles
Enigma 813: Easy as ABC
Posted by on 6 March 2023
From New Scientist #1968, 11th March 1995 [link] [link]
The schedule of matches has been drawn up for the next tournament between Albion, Borough, City, Rangers and United, in which each team will play each of the other teams once. Two matches will take place on each of five successive Saturdays, each of the five teams having one Saturday without a match.
Two of the five teams will be meeting their four opponents in alphabetical order. Given this information you could deduce the complete schedule of matches if I told you either one of the matches scheduled for the first Saturday.
1. Which teams will be meeting their opponents in alphabetical order?
2. Which matches are scheduled for the first Saturday?
[enigma813]
Enigmatic Code 
This Python program collects possible schedules where exactly 2 of the teams meet their opponents in alphabetical order, and counts the number of times each first week match occurs in a schedule.
We then look through the schedules looking for those where both first week matches only correspond to one schedule. And in this case if either of the first week matches was given, we could determine the entire schedule.
The run time is 82ms. (Internal runtime is 16.0ms).
Run: [ @replit ]
from enigma import (defaultdict, subsets, intersect, map2str, join, printf) # construct the matches matches = set(x + y for (x, y) in subsets('ABCRU', size=2)) # construct possible orderings for matches def generate(ms, ws=[]): if not ms: yield ws else: # choose 2 matches for this week for xs in subsets(ms, size=2): if intersect(xs): continue yield from generate(ms.difference(xs), ws + [sorted(xs)]) # find teams that meet their opponents in alphabetical order def ordered(ms): d = defaultdict(list) for w in ms: for (x, y) in w: d[x].append(y) d[y].append(x) for (k, vs) in d.items(): if vs == sorted(vs): yield k # collect possible schedules ss = list() # and count the occurrences of the first week matches m = defaultdict(int) # consider possible matches for ms in generate(matches): # find teams that meet their opponents in alphabetical order ts = list(ordered(ms)) if len(ts) != 2: continue ss.append(ms) for g in ms[0]: m[g] += 1 # find schedules with unique first week matches for ms in ss: if not all(m[g] == 1 for g in ms[0]): continue # output solution printf("{ms}", ms=map2str(enumerate((join(m, sep="+") for m in ms), start=1), arr=": ")) ts = list(ordered(ms)) printf("-> (1) {ts}", ts=join(ts, sep=" ")) printf("-> (2) {w0}", w0=join(ms[0], sep="+")) printf()Solution: (1) City and United meet their opponents in alphabetical order. (2) The first week matches are: Albion vs City; Borough vs Rangers.
The matches are:
So the schedules for each team are: