Forge Practice
179 free coding challenges, solved right in your browser
A free alternative to LeetCode-style practice sites. Every challenge is original, runs in Python or JavaScript inside your browser, and is checked against visible and hidden tests. Each one sits inside the article that teaches its technique, so if you get stuck, the explanation is right above the editor. Hints and full solutions included. No account, no sign-up, nothing to install.
A study plan that works
Work top to bottom: arrays and strings, then linked lists, stacks and queues, trees, graphs, sorting and searching, then dynamic programming. Do every Easy in a section before its Mediums. Read the article first if the technique is new, try the challenge without hints for 20 minutes, then use one hint at a time. Your solved challenges are remembered in this browser and ticked below.
Solved in this browser: 0 / 179
Arrays & Strings (14)
Rotate the On-Call Rota
Easyarray data structure
Peak Balance So Far
Easyarray data structure
Count Pairs Within Budget
Mediumtwo pointer technique
Longest Run of Unique Error Codes
Mediumsliding window technique
Payment Batch Settlement Audit
Mediumprefix sum array
Peak Payout Window
Mediumkadanes algorithm
Three-Tier Payment Risk Partition
Mediumdutch national flag algorithm
Compress Repeated Log Bursts
Mediumstring manipulation patterns
Detecting Anagrams and Palindromes in a Word List
Easyanagram palindrome problems
Spiral Sensor Sweep on a Non-Square Grid
Mediummatrix traversal patterns
Rotate the Batch Log Window
Mediumrotate array problem
Merge Payment Processing Windows
Mediummerge intervals problem
Next Batch Rotation
Hardnext permutation algorithm
Refund Ledger Maximum Product
Mediummaximum product subarray
Linked List (10)
Walk the Tab History Without Losing the Head
Easysingly linked list
Repair the Backward Chain
Mediumdoubly linked list
Round-Robin Quantum Drain
Mediumcircular linked list
Rewire the Audit Trail
Mediumreverse linked list
Audit Log Cycle Entry
Mediumdetect loop linked list
Merge Two Sorted Ledger Streams
Mediummerge sorted linked lists
Middle of a Sensor Stream
Easyfind middle linked list
Drop the Nth Log From the Tail
Mediumremove nth node end
Replay the Cache Eviction Log
Hardlru cache implementation
Clone the Ledger with Random Audit Links
Hardclone linked list random pointers
Stack & Queue (8)
Guarded Undo on the Audit Log
Easystack data structure
Drain the Payment Queue
Easyqueue data structure
Payment Burst Priority Drain
Mediumpriority queue heap
Fraud Burst Detection Window
Mediumdeque double ended queue
Settlement Burst Windows
Hardmonotonic stack pattern
Audit the Bracket Ledger
Easybalanced parentheses problem
Next Higher Quote Alert
Mediumnext greater element problem
Trade Tape Stack Replay
Mediumimplement stack using queue
Trees (15)
Audit the Payment Tree
Mediumbinary tree
The Sorted Insert Trap
Mediumbinary search tree
Postorder Rollback Ledger
Mediumtree traversals
Floor-by-Floor Headcount
Mediumlevel order traversal binary tree
Warehouse Shelf Height
Easyheight depth binary tree
AVL Height Audit After Bulk Inserts
Hardavl tree
Audit the Ledger Tree After Inserts
Hardred black tree
Peak Restock Load Over a Warehouse Aisle
Hardsegment tree
Ledger Delta Audit
Hardfenwick tree binary indexed tree
Case-Sensitive Autocomplete Failure
Hardtrie data structure
Friend Suggestion Common Ancestor
Hardlowest common ancestor
Longest Cable Run in a Network Tree
Mediumdiameter binary tree
Balanced Price Index
Easyconvert bst sorted array
Recover the Payment Ledger Tree
Hardserialize deserialize binary tree
Audit Chain Without a Stack
Hardmorris traversal binary tree
Graphs (17)
Warehouse Conveyor Reachability
Mediumgraph representation
Shortest Settlement Route
Mediumbfs breadth first search
Iterative DFS Reachability in a Service Mesh
Mediumdfs depth first search
Cheapest Settlement Route with Stale Heap Entries
Harddijkstra algorithm
Settlement Ledger Negative Cycle Audit
Hardbellman ford algorithm
Payment Hub Settlement Paths with Overflow Guard
Hardfloyd warshall algorithm
Deploy Order or Deadlock?
Hardtopological sort
Ledger Account Merges and Reachability
Hardunion find disjoint set
Duplicate Edges Break the Edge Count
Hardminimum spanning tree
Settlement Cycle Audit
Mediumcycle detection graph
Split the Payment Network
Mediumbipartite graph check
Warehouse Pallet Clusters
Mediumnumber of islands problem
Deploy Order from Circular Dependencies
Hardstrongly connected components
Heuristic Multiplier Audit
Harda star search algorithm
Clearing House Settlement Throughput
Hardmaximum flow edmonds karp
Audit Trail Reconstruction
Mediumeulerian path circuit
Substation Single Points of Failure
Hardarticulation points bridges
Sorting (8)
Bubble Sort the Morning Warehouse Queue
Easybubble sort
Warehouse Bin Reordering by Minimum Swaps
Easyselection sort
Insertion Sort a Payment Ledger
Easyinsertion sort
Settle Payment Batches in Order
Mediummerge sort
Pivot Scan Count
Mediumquick sort
Heap Sort the Trade Tape
Mediumheap sort
Ledger Digit Sort
Mediumcounting sort radix sort
Pick the Right Sort for a Nearly-Sorted Ledger
Easysorting algorithm comparison
Searching (8)
Find the First Failing Build
Easybinary search algorithm
Throttle Window Capacity
Hardbinary search on answer
Warehouse SKU Scan — First Match Wins
Easylinear search algorithm
Peak Load Window in a Rolling Metric Log
Hardternary search algorithm
Jump Search on Sorted Log Timestamps
Easyjump search algorithm
Sensor Probe Estimator
Mediuminterpolation search
Fraud Window: Exponential Probe in a Sorted Ledger
Mediumexponential search
Two-Sided Route Meet
Mediumbidirectional search
Dynamic Programming (15)
Warehouse Restock Runs
Mediumintroduction dynamic programming
Ledger Runway: Tabulated Max Payout
Mediummemoization vs tabulation
Ledger Batch Cap: Reconstruct the Winning Batch
Hard01 knapsack problem
LCS Diff Reconstruction with Deterministic Tie-Breaking
Hardlongest common subsequence
Payment Burst Streak
Hardlongest increasing subsequence
ATM Note Dispenser Minimum Count
Hardcoin change problem
Cheapest Payment Batch Merge Order
Hardmatrix chain multiplication
Ledger Reconciliation by Edit Distance
Hardedit distance problem
Fiber-Optic Splice Revenue
Hardrod cutting problem
Warehouse Pallet Stacking
Easyfibonacci dynamic programming
Ledger Settlement: Can These Invoices Clear Exactly?
Hardsubset sum problem
Canary Rollout Floor Budget
Hardegg drop problem
Ledger Token Audit
Hardword break problem
Ledger Palindrome Audit
Hardpalindrome partitioning
Payment Network Reroot: Total Distance to All Nodes
Harddynamic programming trees
Hashing (11)
First Repeated Transaction ID
Easyhash table hash map
Warehouse Slot Probing
Mediumhash collisions resolution
Find the Duplicate Charge Pair
Easytwo sum problem hashing
Batch Warehouse SKUs by Anagram Signature
Mediumgroup anagrams problem
Longest Login Streak from Session IDs
Mediumlongest consecutive sequence
Duplicate Log Window Detector
Hardrabin karp string matching
Two-Shelf Warehouse Slotting
Mediumcuckoo hashing
SHA-256 Padding Planner
Easysha 256 hashing
Count MD5 Padding Blocks Before Hashing
Mediummd5 hashing algorithm
Adversarial Hash Seed Audit
Harduniversal hashing
ECC Memory Syndrome Triage
Mediumhamming code error detection
Greedy & Backtracking (13)
Booking the Most Meeting Rooms
Mediumgreedy algorithm introduction
Earliest-Finish Job Batch
Mediumactivity selection problem
Decode a Corrupt Huffman Stream
Hardhuffman coding
Warehouse Pallet Stacking
Mediumbacktracking introduction
Queen Seat Audit: Count Non-Attacking Layouts
Hardn queens problem
Ledger Grid Completion
Hardsudoku solver
Warehouse Robot Route Audit
Mediumrat in maze problem
Dispatch Route Permutations with Duplicate Stops
Hardpermutations backtracking
Bandwidth Slice Planner
Mediumfractional knapsack greedy
Freelancer Deadline Slot Assignment
Mediumjob scheduling greedy
Canonical Huffman Code Lengths from Symbol Frequencies
Mediumhuffman encoding greedy
Payment Batch Windows
Mediuminterval scheduling problem
Courier Loop Fuel Plan
Mediumgas station problem
Complexity Analysis (4)
Recursion (9)
Symlink Chain Depth
Easyintroduction recursion
The Ledger That Blew the Stack
Mediumrecursion vs iteration
Replaying the Warehouse Relocation
Mediumtower of hanoi
Ledger Range Total
Mediumdivide and conquer
Ledger Split Audit
Easydivide conquer introduction
Payment Batch Disorder Score
Mediumcounting inversions
Closest Pair of Payment Terminals
Hardclosest pair of points
Strassen Block Multiply (2×2 Base Case)
Hardstrassen matrix multiplication
Warehouse Fence With Collinear Posts
Hardconvex hull algorithms
Strings (8)
Find Every Log Marker
Easystring searching algorithms
KMP Log Pattern Scanner
Mediumkmp string matching
Log Signature Scan with Z-Array
Mediumz algorithm string matching
Boyer-Moore Log Signature Scan
Mediumboyer moore string search
Palindrome Audit for Log Signatures
Hardmanacher algorithm longest palindrome
Signature Scan With Overlapping Matches
Hardaho corasick algorithm
Longest Repeated Motif in a Sequenced Genome
Hardsuffix array
Duplicate Log Window Detection
Mediumrolling hash string hashing
Number Theory (5)
Scheduling (4)
Data Compression (4)
Numerical Analysis (3)
Cryptography (8)
Decode the ROT13 Audit Trail
Easycaesar cipher
Recover the Repeating Key Length from Intercepted Ledger Traffic
Mediumvigenere cipher
The Padding Oracle at the Payment Gateway
Hardrsa algorithm
Audit the Handshake Shared Secret
Harddiffie hellman key exchange
The Round Key Trap: Verify AES MixColumns Inversion
Hardaes encryption
Toy Curve Public Key
Hardelliptic curve cryptography
Venona Pad Reuse Audit
Mediumone time pad
Reconcile Webhook Signature Wire Formats
Harddigital signatures
Geometry (4)
Linear Algebra (5)
Genetic Algorithms (2)
Quantum Algorithms (4)
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