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Module 02

DSA Basics

Ninety problems that build loop, array and string fluency — from warm-ups to exam-level questions.

Before patterns come fundamentals. These problems are short stories about stalls, classrooms and queues that train you to turn words into loops, counters, prefix sums, two pointers and hash maps. Every problem runs in your browser in C, C++, Java or Python, with hidden tests and reference solutions in all four.

  • Write array and string loops without off-by-one errors
  • Use counting, prefix sums, two pointers and hash maps by reflex
  • Handle edge cases (empty input, duplicates, negatives) the hidden tests check for
  • Solve the typical online-assessment coding question in under 20 minutes
Start with The Juice Stall Takings Unlock everything
90
problems
8
free to start
~38h
of study
01

Warm-up

Single passes, counters and simple conditions. Get comfortable with the editor and the judge.

01 The Juice Stall TakingsFree Single pass · running total Easy
02 Who Cleared the Cut-OffFree Single pass · counting with a condition Easy
03 The Mirror NameplateFree Two pointers from both ends Easy
04 The Tallest Building on the SkylineFree Single pass · running maximum Easy
05 Turn the Queue AroundFree Two pointers · in-place reverse Easy
06 Counting the Vowels in a Slogan Single pass over characters Easy
07 The Runner-Up Score Single pass · two running values Easy
08 Finding the Roll Number on the List Linear search Easy
09 The Backwards Wi-Fi Password Building a string backwards Easy
10 How Many Times Does That Letter Appear Single pass over characters Easy
11 The Lucky Bill Number Character to digit · running total Easy
12 Counting Words in the Notice Single pass with a state flag Easy
13 Is the Token Queue in Order Compare neighbours in one pass Easy
14 Squeeze Out the Spaces Build a filtered string Easy
15 The Week's Temperature Swing Running maximum and minimum Easy
16 Even-Numbered Seats Single pass · counting with a condition Easy
17 Swap the Ends of the Row Direct index access Easy
18 The Shouting Version of the Message Character transformation Easy
19 One Row of the Marks Sheet Indexing into a 2D array Easy
20 Digits Hidden in the Address Character classification Easy
21 The Cricket Team's Batting Product Running product · overflow awareness Easy
22 Above the Class Average Two passes · derive then compare Easy
23 Fixing the Typo in the Poster Character replacement Easy
24 The Longest Word on the Slide Scan words · keep the best Easy
25 The Whole Hall's Attendance Nested loop over a grid Easy
26 Cancel Every Booking of One Slot Build a filtered array Easy
27 Readings from the Odd Minutes Stepping through indexes Easy
28 Shouted Words in the Chat Character classification Easy
29 Is That Word in the Notice Naive substring search Easy
30 Shuffling Two Decks Together Two indexes walking together Easy
02

Building up

Frequency maps, prefix sums, two pointers, sorting and 2-D grids.

01 Push the Empty Boxes to the BackFree Write pointer (stable partition) Easy
02 Two Items for Exactly the Budget Two pointers on a sorted array Easy
03 The One Letter Nobody Repeats Frequency count · second pass Easy
04 Did They Just Jumble My Name Frequency count · compare Easy
05 Read the Address Backwards Split and rebuild Easy
06 The Best Three Overs Fixed-size sliding window Easy
07 Rotate the Duty Roster Reverse three times (rotation) Medium
08 One Entry per Visitor Write pointer on a sorted array Easy
09 The Class Representative Vote Pairing off votes (majority) Medium
10 Runs Between Two Overs Prefix sums Medium
11 Pairs of Friends with the Same Age Gap Hash set lookup Medium
12 The Longest Cheer Run length in one pass Easy
13 Turning the Timetable on Its Side Matrix transpose Medium
14 The Cross on the Chessboard Indexing the diagonals Easy
15 Joining Two Merit Lists Two pointers · merge Medium
16 Looking Up a Sorted Price List Binary search Medium
17 The Best Stretch of Trading Days Running best (Kadane) Medium
18 Short Code for the Chant Run-length encoding Medium
19 Names for the Certificate Word-start detection Easy
20 How Many Different Subjects Hash set · counting distinct values Easy
21 The Missing Answer Sheet Sum formula / arithmetic trick Medium
22 Who Came Back to the Counter First Hash set while scanning Medium
23 The Shared Folder Name Compare characters column by column Medium
24 Is the Slogan a Perfect Mirror Two pointers with filtering Medium
25 Balancing the Luggage Trolley Prefix sum · balance point Medium
26 Ladies' Compartment First Stable partition into a new array Easy
27 Subjects Both Friends Are Taking Hash set intersection Medium
28 The Letter the Chant Loves Most Frequency array · pick the best Medium
29 One Buy, One Sell Running minimum · best so far Medium
30 Rooms with a Sea View Running maximum while scanning Medium
31 The Best Performing SectionFree Best row of a grid Easy
32 How Often Does the Slogan Repeat Naive pattern matching · counting Medium
33 Swap Only the Vowels Two pointers with skipping Medium
34 Did Someone Rotate the Password Rotation check by doubling Medium
35 Two Bills That Settle the Account Hash map of what you have seen Medium
03

Exam level

The kind of problem that decides an online assessment — several ideas combined, tight edge cases.

01 The Longest Stretch of New FacesFree Variable sliding window with last-seen positions Medium
02 Counting the Balanced Shifts Prefix sums in a hash map Hard
03 Everyone Else's Contribution Prefix and suffix products Hard
04 The Shortest Delivery Run Variable sliding window (shrink from the left) Hard
05 The Longest Run of Jersey Numbers Hash set · count runs from their start Hard
06 Quarantine the Spoilt Shelves Mark first, change afterwards Hard
07 Turning the Group Photo Transpose then reverse each row Hard
08 The Watchman's Round Spiral traversal with four boundaries Hard
09 The Longest Mirror Inside the Banner Expand around every centre Hard
10 The Loudest Second of Every Minute Monotonic deque over a sliding window Hard
11 A Thali with at Most k Flavours Sliding window with a count map Hard
12 The Longest Winning Run with k Retakes Sliding window with a budget Hard
13 Where the Jumbled Code Appears Sliding window of letter counts Hard
14 Adding Two Very Long Bills Digit-by-digit addition with a carry Hard
15 Almost a Mirror Word Two pointers with one allowed skip Hard
16 Repainting the Longest Stripe Sliding window with the most common letter Hard
17 Shifts with Exactly k Night Duties Counting windows · at most k minus at most k−1 Hard
18 Seating the Team Together Fixed window over a rearrangement Hard
19 The Smallest Token Nobody Took Placing values at their own index Hard
20 Merging the Overlapping Bookings Sort by start, then sweep Hard
21 The Best k by k Plot 2D prefix sums Hard
22 The Word the Notice Repeats Most Hash map of word counts Medium
23 Sorting the Letters by Popularity Counting sort by frequency Hard
24 Rain on the Uneven Roof Two pointers with running maximums Hard
25 The Shortest Notice That Says It All Sliding window with a need count Hard