Add an exam questions page, by level from 1 to 4

[ethan42]

Adds a page with every exam question (108) sorted into four levels, so you can practise from easiest to hardest:

• New page: questions/exam-questions.md, generated by tools/gen-exercises.py and linked from the home page and the question bank. Each entry opens the question's own page on the study site (statement + hint), not the exam PDF.
• Question pages: exam questions now show «Επίπεδο N/4» and a navigation row with previous and next links within the same level. The order comes from the generated _data/exams.yml.
• Scale: a new level field (1–4), only on exam questions; the guide-wide ★ difficulty is unchanged. The rubric is in STYLE.md:
  1. short question, not a whole program (trace or explain code, short function)
  2. simple complete program reading stdin or argv, one loop or a simple double loop (usually Θέμα 3)
  3. data structures and heap memory (lists, trees, malloc/free)
  4. the hardest θέμα, usually the last one (files, dynamic programming, non-obvious algorithms)
• Lint: exam questions must have a level, and other questions must not have one.

Counts: level 1: 14 · level 2: 53 · level 3: 25 · level 4: 16.

Ratings to review

I rated each question by reading it against the rubric. Position in the exam was a hint, but the content decided. Edit level: in any question file and run tools/gen-exercises.py to regenerate.

Ratings that go against the position hint:

• Θέμα 3 rated 4: exam-2023-dec-q3 (dynamic programming), exam-2024-dec-q3 (3-sum faster than O(n³)), exam-2024-jul-q3 (greedy algorithm with a proof).
• Θέμα 1 rated 2: exam-2023-dec-q1, exam-2024-dec-q1 and exam-2024-jul-q1 are complete programs that read their input.
• Last θέμα rated below 4: exam-2026-jan-q6 is 1 (explain a NULL dereference) and exam-2024-jul-q4 is 3 (tree traversal).
• Borderline: exam-2026-jun-q3 (stonks, 2 or 3?) and exam-2026-jun-q4 (scoreboard, 3 or 4?).

2023 online exam: every θέμα is a whole program, so there is no level 1.

• In four variants the levels don't rise from θέμα 1 to θέμα 4, because the hard one is θέμα 2 or 3: ex7 (2,2,4,2), ex11 (2,2,3,2), ex13 (2,3,3,2) and ex15 (2,2,3,2).
• Borderline: ex12-q4 and ex13-q4 (2 or 4?), ex4-q3 (2 or 3?), and ex5-q4, ex10-q4 and ex13-q3 (3 or 4?).

All regular exams (44)

id	Τίτλος	★	Μονάδες	Επίπεδο	Γιατί
exam-2023-dec-q1	Κάντο όπως ο Βιετά	★★☆	-	2	argv count, one loop computing Viète product
exam-2023-dec-q2	Εαυτοί Αριθμοί	★★☆	-	2	argv range, simple double loop over digits
exam-2023-dec-q3	Σκαλί-Σκαλί	★★☆	-	4	stair counting needs dynamic programming, complexity bound, overflow
exam-2023-dec-q4	Το Νερό Νεράκι	★★★	-	4	trapping rain water in O(n), minimal memory
exam-2024-dec-q1	Προσεγγίζοντας το π	★☆☆	-	2	complete argv program, one loop Wallis product
exam-2024-dec-q2	Οι Καλύτεροι Αριθμοί, Παμψηφεί	★★☆	-	2	argv range, double loop digit check
exam-2024-dec-q3	Η Τριπλέτα Στόχος	★★★	-	4	3-sum faster than O(n^3): sort + two pointers
exam-2024-dec-q4	Λύσε τον Λαβύρινθο	★★★	-	4	maze path down/right, DP over grid from stdin
exam-2024-jul-q1	ASCII Encoding	★☆☆	-	2	stdin character loop, hex output
exam-2024-jul-q2	Στατιστικές	★★☆	-	2	argv ints, mean and std deviation
exam-2024-jul-q3	Βέλτιστη Μοιρασιά Πίτσας	★★★	-	4	greedy with sorting, prove optimality, complexity
exam-2024-jul-q4	Reverse Inorder Traversal	★★☆	-	3	recursive reverse inorder on binary tree
exam-2024-sep-q1	Η συνάρτηση about	★☆☆	5	1	short function printing three lines
exam-2024-sep-q2	Η συνάρτηση what	★☆☆	10	1	trace binary search, explain function
exam-2024-sep-q3	Εύρεση μηδενός σε πίνακα	★★☆	15	1	short function: binary search on sorted array
exam-2024-sep-q4	Μετρητής λέξεων	★★☆	25	2	stdin word count, one loop with state
exam-2024-sep-q5	Αντιστροφή λίστας	★★☆	25	3	reverse linked list into new heap-allocated list
exam-2024-sep-q6	Γεμίζοντας με χρώμα	★★★	25	4	flood fill from file via argv, recursion/BFS
exam-2025-jan-q1	Mystery	★☆☆	10	1	trace loop with shifts on given input
exam-2025-jan-q2	Η συνάρτηση dog	★☆☆	10	1	explain/trace string concatenation function
exam-2025-jan-q3	Κινούμενος Μέσος Όρος - sma	★★☆	25	2	argv ints, moving average over last 10
exam-2025-jan-q4	Αθροιστής Δέντρων - sumtree	★☆☆	20	3	recursive sum over binary tree
exam-2025-jan-q5	Συνένωση Αλφαριθμητικών - join	★★☆	20	3	join strings into heap-allocated result
exam-2025-jan-q6	Μετρώντας τα Αστέρια - stars	★★★	25	4	file grid, 2D prefix sums for repeated queries
exam-2025-sep-q1	Mystery	★☆☆	10	1	trace digit-sum loop
exam-2025-sep-q2	Η συνάρτηση transform	★☆☆	15	1	explain/trace in-place string reverse
exam-2025-sep-q3	Μέση Τιμή Τυχαίων Μεταβλητών - mean	★☆☆	25	2	argv value/probability pairs, one loop
exam-2025-sep-q4	Μεσαίο Στοιχείο Λίστας	★★☆	25	3	middle of linked list, new heap node
exam-2025-sep-q5	Το Καλό το Μονοπάτι - path	★★★	25	4	min-cost grid path DP from file
exam-2026-jan-q1	Mystery	★☆☆	10	1	trace loop with bit masking
exam-2026-jan-q2	Η συνάρτηση compute	★★☆	15	1	explain/trace merge, memory class, spot bug
exam-2026-jan-q3	Εύρεση Πρώτων Παραγόντων - factor	★★☆	25	2	argv int, prime factorisation loop
exam-2026-jan-q4	Διερμηνέας Αριθμητικών Εκφράσεων - eval	★★☆	25	3	recursive evaluation of expression tree
exam-2026-jan-q5	Περικύκλωση - encirclement	★★★	25	4	grid reachability/flood fill from stdin, error handling
exam-2026-jan-q6	Ο Στέργιος Ξαναχτυπά	★☆☆	10	1	reason about NULL dereference outcome, short
exam-2026-jun-q1	Η συνάρτηση mystery	★☆☆	15	1	trace shift loop
exam-2026-jun-q2	Η συνάρτηση paws	★★☆	15	1	explain/trace reverse, find off-by-one bug
exam-2026-jun-q3	Επενδύσεις στο Χρηματιστήριο	★★☆	45	2	stdin values, min/max plus one-pass best profit
exam-2026-jun-q4	World Cup 2026	★★★	35	4	parse match file, aggregate teams, multi-key sort
exam-2026-sep-q1	Mystery	★☆☆	10	1	trace digit-reverse loop
exam-2026-sep-q2	Η συνάρτηση cons	★☆☆	15	1	explain/trace run length, spot off-by-one
exam-2026-sep-q3	Το μεγαλύτερο άλμα - polevault	★★☆	25	2	argv name/score pairs, find maximum
exam-2026-sep-q4	Ν-οστό Στοιχείο Λίστας	★★☆	25	3	n-th element of linked list into new node
exam-2026-sep-q5	Η Μεγαλύτερη Χωρητικότητα - capacity	★★★	25	4	max container area, two-pointer O(n)

2023 online exam (64)

id	Τίτλος	★	Μονάδες	Επίπεδο	Γιατί
exam-2023-fall-ex0-q1	Ραβασάκι	★☆☆	25	2	stdin echo appending a suffix per line
exam-2023-fall-ex0-q2	Ταιριαστές Καρδιές	★★☆	25	2	argv strings, balanced-hearts check in one loop
exam-2023-fall-ex0-q3	Καλύτερο Ταίριασμα	★★★	25	3	parse file into structs, store dynamically and sort
exam-2023-fall-ex0-q4	Αγαπήσιμοι Αριθμοί	★★☆	25	4	palindromic cubes up to 1e18: enumerate cubes, 64-bit
exam-2023-fall-ex1-q1	Δεκαεξαδικοί	★☆☆	25	2	stdin bytes printed as hex, one loop
exam-2023-fall-ex1-q2	Εντοπισμός Διπλών Ορισμάτων	★★☆	25	2	argv duplicates, simple double loop
exam-2023-fall-ex1-q3	Περιστροφή Πίνακα	★★☆	25	3	matrix from file into heap 2D array, rotate
exam-2023-fall-ex1-q4	Debugging	★★☆	25	3	debug crashing linked-list program with malloc/free
exam-2023-fall-ex2-q1	Pikaκίστικα	★★☆	25	2	stdin word prefixing, one loop
exam-2023-fall-ex2-q2	Τουρνουά	★☆☆	25	2	argv all ordered pairs, double loop
exam-2023-fall-ex2-q3	Μίνι Βάση Δεδομένων	★★★	25	3	parse file records into structs, filter and sort by date
exam-2023-fall-ex2-q4	Ανεβαίνοντας Επίπεδο	★★☆	25	4	count level-up ways to 50: dynamic programming, 64-bit
exam-2023-fall-ex3-q1	Προσθήκη Νιφάδων	★★☆	25	2	stdin, insert character between letters
exam-2023-fall-ex3-q2	Φορτωμένο Έλκηθρο	★★☆	25	2	argv triples summing to target, nested loops
exam-2023-fall-ex3-q3	Εύρεση Λέξεων	★★☆	25	2	read word file line by line, substring filter
exam-2023-fall-ex3-q4	Χτίζοντας έναν Χιονάνθρωπο	★★★	25	4	file input, minimum-cost placement needs median insight
exam-2023-fall-ex4-q1	Αναγράμματα	★★☆	25	2	two argv strings, letter counts for anagram check
exam-2023-fall-ex4-q2	Τα Πάνω Κάτω	★★☆	25	2	stdin word replacement up/down
exam-2023-fall-ex4-q3	Rickroll Τριπλέτες	★★★	25	2	brute force over exponents with unsigned 32-bit wraparound
exam-2023-fall-ex4-q4	Αλλαγή Μεγέθους	★★☆	25	2	binary file copy replacing two bytes at offset
exam-2023-fall-ex5-q1	Αναποδογύρισμα	★☆☆	25	2	stdin, reverse each line
exam-2023-fall-ex5-q2	Μαγικό Ζευγάρι	★★☆	25	2	argv digit counts comparison
exam-2023-fall-ex5-q3	Επιλογή	★★☆	25	3	file records stored dynamically and sorted by score
exam-2023-fall-ex5-q4	Μεταμορφώσιμες Προτάσεις	★★★	25	3	split sentences into word arrays, bijective word mapping
exam-2023-fall-ex6-q1	Μέγιστος Κοινός Διαιρέτης	★☆☆	25	2	argv validation and Euclid gcd loop
exam-2023-fall-ex6-q2	Μήνυμα από τον Καίσαρα	★☆☆	25	2	stdin ROT13 character mapping
exam-2023-fall-ex6-q3	Σπάσε το PIN	★★☆	25	2	brute-force 4-digit pins against given checker code
exam-2023-fall-ex6-q4	Ταξινόμηση Αρχείων Καταγραφής	★★☆	25	3	store all log lines dynamically, sort by timestamp
exam-2023-fall-ex7-q1	Ανάβεις Φωτιές	★☆☆	25	2	stdin, prefix emoji on lines of any length
exam-2023-fall-ex7-q2	Κρυφό Μήνυμα	★☆☆	25	2	xor two files byte by byte
exam-2023-fall-ex7-q3	Δίδυμοι Πρώτοι	★★★	25	4	twin primes over huge ranges: (segmented) sieve, heap array
exam-2023-fall-ex7-q4	Κόψιμο Αρχείων	★☆☆	25	2	copy file except last N bytes
exam-2023-fall-ex8-q1	Άρτια Bits	★☆☆	-	2	argv integers, count set bits
exam-2023-fall-ex8-q2	Αποκωδικοποίηση	★★☆	-	2	stdin hex pairs decoded to characters
exam-2023-fall-ex8-q3	Πολλαπλασιασμός Πινάκων	★★☆	-	3	two matrices from files into heap arrays, multiply
exam-2023-fall-ex8-q4	Ταξινόμηση Πακέτων	★★★	-	3	store packets dynamically from file, sort by id
exam-2023-fall-ex9-q1	Έλεγχος Εκτελέσιμου	★☆☆	-	2	check first four bytes of a file
exam-2023-fall-ex9-q2	Πετυχαίνοντας τον Στόχο	★★☆	-	2	argv triples summing to goal, nested loops
exam-2023-fall-ex9-q3	Πολύτιμοι Πίνακες	★★☆	-	3	matrix from file into heap array, row/column sums
exam-2023-fall-ex9-q4	Κάδρο	★★★	-	3	store unbounded stdin lines on heap to frame them
exam-2023-fall-ex10-q1	Αφαίρεση Μη Λατινικών Χαρακτήρων	★★☆	-	2	stdin character filter, bold first word
exam-2023-fall-ex10-q2	Ασημένιο Κλάσμα	★★☆	-	2	continued fraction in a loop, double precision
exam-2023-fall-ex10-q3	Κρυμμένο Μήνυμα	★★☆	-	3	store all file words dynamically, index by argv positions
exam-2023-fall-ex10-q4	Πλησιάζοντας στον Στόχο	★★☆	-	3	argv points into struct array, sort by distance
exam-2023-fall-ex11-q1	Αναζητώντας τον Blinky	★★☆	-	2	stdin whole-word match and decoration
exam-2023-fall-ex11-q2	Τοποθέτηση του Pacman	★★☆	-	2	grid positions avoiding ghost rows/columns, double loop
exam-2023-fall-ex11-q3	Κινήσεις σε Πλέγμα	★★☆	-	3	grid from file into heap 2D array, torus simulation
exam-2023-fall-ex11-q4	Clyde Πρώτοι	★★☆	-	2	primality of digit rotations, simple loops
exam-2023-fall-ex12-q1	Γραμμή και Γράμμα	★☆☆	-	2	stdin lines merged into one word
exam-2023-fall-ex12-q2	Προσεγγίζοντας την Λέξη	★★☆	-	2	argv wordle-style letter matching, double loop
exam-2023-fall-ex12-q3	Κρεμάλα	★★☆	-	2	filter dictionary file against hangman pattern
exam-2023-fall-ex12-q4	Εαυτοί Αριθμοί	★★☆	-	2	self numbers in range with digit-sum loop
exam-2023-fall-ex13-q1	Κάντο όπως ο Βιετά	★★☆	-	2	Viete product in a loop, sqrt
exam-2023-fall-ex13-q2	Αλλαγή Τέρματος	★☆☆	-	3	stdin grid of given size into heap array, rotate 180
exam-2023-fall-ex13-q3	Πρωτάθλημα	★★★	-	3	aggregate teams from file into structs, sort with tie-break
exam-2023-fall-ex13-q4	Τυχεροί Αριθμοί	★★☆	-	2	sum of two cubes: loop with cube-root check
exam-2023-fall-ex14-q1	Ορεκτικό	★☆☆	-	2	stdin, emoji at both ends of each line
exam-2023-fall-ex14-q2	Μένοντας στις Σωστές Θερμίδες	★★☆	-	2	sort argv calories, greedy count
exam-2023-fall-ex14-q3	Μετρήσεις Θερμοκρασίας	★★☆	-	3	store file measurements dynamically, sort by distance to mean
exam-2023-fall-ex14-q4	Τρόποι να Φάμε Παϊδάκια	★★★	-	4	count eating orders: dynamic programming / recursion
exam-2023-fall-ex15-q1	Πλαγιαστά Γράμματα	★☆☆	-	2	stdin, italic letters between asterisks
exam-2023-fall-ex15-q2	Τυπώνοντας τον Πίνακα ASCII	★★☆	-	2	ASCII table in columns, nested loops
exam-2023-fall-ex15-q3	Ταξινομώντας τα Άλματα	★★★	-	3	parse JSON-like file into structs, sort descending
exam-2023-fall-ex15-q4	Δυνατότητες	★★☆	-	2	dictionary file words matched to phone digits

Verified:

• make check passes.
• Running gen-exercises.py a second time changes nothing.
• The lint rule fails both ways: a slide question with a level, and an exam question without one.
• A local build with the same Jekyll image as CI, after site-prep.py:
  • all 108 links resolve and the four anchors exist
  • the page title is «Θέματα εξετάσεων»
  • previous/next work, and the first question of a level has no previous link

CI fix (separate commit): the Check PDFs step now runs apt-get update before installing poppler-utils. The runner's stale package index made that install 404 twice, a failure unrelated to this change.

Opened from socrates.