Every public article in this language appears here: current work, reviewed notes, curated sections, and preserved historical material. Homepage sections and Notes are curated discovery surfaces; this archive preserves the complete, route-stable record.
I ran an AI-agent pipeline in Japan's Ministry of Internal Affairs and Communications nationwide CTF. The official result was 26th place with 5,701 points. The bigger lesson was not model capability, but ownership, shared state, verification, and control-plane design.
How a historical Jekyll site used GitHub Actions to validate every branch and pull request while reserving production publishing for changes merged to main.
Archived technical note: BOJ 10803 — Making Squares: A Verified Recurrence Bound.
A radix-2 FFT guide: DFT normalization, butterfly decomposition, polynomial convolution, and the limits of floating-point integer results.
Archived technical note: BOJ 1199 — Euler Circuit.
A proof that every bounded sequence in finite-dimensional Euclidean space has a convergent subsequence, with the compactness distinction.
The monotone convergence theorem for real sequences, proved from the least-upper-bound property and illustrated with examples.
The nested interval theorem: completeness gives a common point, while shrinking interval lengths make it unique.
Archived technical note: BOJ 7579 — App.
A grouped reference to algebraic, metric, order, completeness, and limit concepts used across introductory analysis.
A compact, grouped reference to 19 foundational ideas in sets, algebraic structures, functions, and cardinality, with examples.
Why the gcd of two integers is an integer linear combination, with a least-positive-combination proof and the field-only polynomial analogue.
Archived technical note: 백준 1006번 - 습격자 초라기(증명).
Archived technical note: 백준 1035번 - 조각 움직이기.
Archived technical note: 백준 1395번 - 스위치.
Archived technical note: 백준 19565번 - 수열 만들기.
Archived technical note: 백준 3653번 - 영화 수집.
A proof and worked trace of the Euclidean algorithm, including its nonnegative-input C++ implementation and logarithmic step count.
Learn feasible flows, residual forward and reverse arcs, augmenting paths, cut bounds, and when Ford–Fulkerson terminates.
Derive Floyd–Warshall’s all-pairs recurrence, implement it safely in C++, and interpret unreachable pairs and negative cycles.
A proof of the max-flow min-cut theorem using residual reachability, with a concrete flow-and-cut certificate.
Archived technical note: BOJ. Cubing (5373).
Archived technical note: BOJ 7469 - K-th Number.
Archived technical note: BOJ 1520 - Downhill Paths.
Archived technical note: BOJ. Match The Numbers (2494).
Archived technical note: BOJ. Square (7626).
Archived technical note: BOJ. Line Group (2162).
Archived technical note: BOJ. Crossing Lines 3 (20149).
Archived technical note: BOJ 1069 - Going Home.
Archived technical note: BOJ. Two Circles (7869).
Archived technical note: BOJ. Crossing Lines (17386).
Archived technical note: BOJ. Crossing Lines 2 (17387).
Archived technical note: BOJ. The Area Of The Polygon (2166).
Archived technical note: BOJ. Passionate Gang-Ho 4 (11378).
Archived technical note: BOJ. Maximum Flow (6086).
Archived technical note: BOJ. Passionate Gang-Ho 2 (11376).
Archived technical note: BOJ. Passionate Gang-Ho (11375).
Archived technical note: BOJ. Palindrome Is Not A Palindrome (15927).
Archived technical note: BOJ. Allocating Cattle Shed (2188).
Archived technical note: BOJ. Round Trip From City To City (17412).
Archived technical note: BOJ. String And Query (13713).
Archived technical note: BOJ. Repeated Pattern (16229).
Archived technical note: AtCoder. ABC 238 C 1111gal password.
Archived technical note: AtCoder. ABC 238 D ABC Transform.
Archived technical note: BOJ. Report Problem No.15164 (16163).
Archived technical note: BOJ. Find Kth Shortest Path (1854).
Archived technical note: BOJ. Ship (1092).
Archived technical note: BOJ. Sensor (2212).
Archived technical note: BOJ. Castle Difense (17135).
Archived technical note: BOJ. Castle Difense (2473).
Archived technical note: BOJ. Puyo Puyo (11559).
Archived technical note: BOJ. King Of Fishing (17143).
Archived technical note: BOJ. Synthesize Solutions (14921).
Archived technical note: BOJ. Convey News (1135).
Archived technical note: BOJ. Make A Big Number (16496).
Archived technical note: BOJ. Cheese (2638).
Archived technical note: BOJ. Two-Dimenstion Array And Operation (17140).
Archived technical note: BOJ. CCW (11758).
Archived technical note: Tree Investment.
Archived technical note: BOJ. Four Numbers Of With Sum (7453).
Archived technical note: BOJ. Lecture Room Assignment (11000).
Archived technical note: BOJ. N th Biggest Number (2075).
Archived technical note: LeetCode. 1028. Recover a Tree From Preorder Traversal.
Archived technical note: BOJ. Race (2517).
Archived technical note: BOJ. Candy Box (2243).
Archived technical note: BOJ. Manipulate Ladder (15684).
Archived technical note: BOJ. Bind A Number (1744).
Archived technical note: BOJ. Scale (2437).
Archived technical note: BOJ. Perfect Binary Tree (9934).
Archived technical note: BOJ. Boggle (9202).
Archived technical note: BOJ. Save The Sheep (16437).
Archived technical note: BOJ. Distance Between Vertices (1761).
Archived technical note: BOJ 1240 — Distance Between Nodes.
Archived technical note: BOJ. Cheese (2636).
Archived technical note: BOJ. Is This Tree? (6416).
Archived technical note: BOJ. Dragon Curve (15685).
Archived technical note: BOJ. Factory (7578).
Archived technical note: BOJ. Company Culture 1 (14267).
Archived technical note: BOJ. Binary Tree (13325).
Archived technical note: BOJ. Tree (4256).
Archived technical note: BOJ. Height And Width Of Tree (2250).
Archived technical note: BOJ. Slope (14890).
Archived technical note: BOJ. LCA (11437).
Archived technical note: BOJ. Phone Number List (5052).
Archived technical note: Memo. To prove.
Archived technical note: BOJ 17144 — Fine Dust Simulation.
Archived technical note: BOJ 1339 - Word Math.
Archived technical note: BOJ 2169 — Robot Control.
Archived technical note: BOJ 15683 — CCTV Surveillance.
Archived technical note: BOJ. Population Movement (16234).
Archived technical note: BOJ. Marble Escape 2 (13460).
Archived technical note: BOJ. Tetromino (14500).
Archived technical note: BOJ 14503 — Robot Vacuum (14503).
Archived technical note: BOJ. Snake (3190).
Archived technical note: BOJ. Roll The Dice (14499).
Archived technical note: BOJ. Baby Shark (16236).
Archived technical note: BOJ. Chicken Delivery (15686).
Archived technical note: BOJ. Longest Palindrome Substring (13275).
Archived technical note: BOJ. Reason the fox came up to Island (17131).
Archived technical note: LeetCode. 43. Multiply Strings.
Archived technical note: BOJ. Minimum Value (10999).
Archived technical note: BOJ. Histogram (1725).
Archived technical note: BOJ. Sum of numbers 7 (2268).
Archived technical note: BOJ. Coffe Shop (1275).
Archived technical note: BOJ. Minimum Value (10868).
Archived technical note: BOJ. Water taxi (2836).
Archived technical note: BOJ. Northwesterly Wind (5419).
Archived technical note: BOJ. line Drawing (2170).
Archived technical note: BOJ. ATM (4013).
Archived technical note: Codeforces Round 771 (Div. 2) A. Reverse.
Archived technical note: Codeforces Round 771 (Div. 2) B. Odd Swap Sort.
Archived technical note: Codeforces Round 771 (Div. 2) C. Inversion Graph.
Archived technical note: Codeforces Round 771 (Div. 2) D. Big Brush.
Archived technical note: BOJ. 2-SAT - 4 (11281).
Archived technical note: AtCoder. Regular Contest 135 A Floor, Ceil Decomposition.
Archived technical note: AtCoder. Regular Contest 135 B Sum of Three Terms.
Archived technical note: AtCoder. Regular Contest 135 C XOR to All.
Archived technical note: Codeforces Global Round 19 B. MEX and Array.
Archived technical note: BOJ. Idol (3648).
Archived technical note: Codeforces Global Round 19 C. Andrew and Stones.
Archived technical note: Codeforces Global Round 19 A. Sorting Parts.
Archived technical note: LeetCode. 100. Same Tree.
Archived technical note: BOJ. 2-SAT - 3 (11280).
Archived technical note: BOJ. Tree and Query 2 (13511).
Archived technical note: BOJ. Soccer Tactics (3977).
Archived technical note: BOJ. Domino (4196).
Archived technical note: BOJ. Palindrome Devision (1509).
Archived technical note: BOJ. Find Minimum (11003).
Archived technical note: BOJ. Road Network (3176).
Archived technical note: BOJ. LCA 2 (11438).
Archived technical note: BOJ. Composite Function And Query (17435).
Archived technical note: BOJ. Lowest Common Ancestor (3584).
Archived technical note: BOJ. SeongYeon Park (1086).
Archived technical note: Codeforces Round 770 (Div. 2) A. Reverse and Concatenate.
Archived technical note: Codeforces Round 770 (Div. 2) B. Fortune Telling.
Archived technical note: BOJ. Traveling Salesman problem (2098).
Archived technical note: BOJ. RGB Distance 2 (17404).
Archived technical note: BOJ. Final Ranking (3665).
Archived technical note: BOJ. Workbook (1766).
Archived technical note: BOJ. Distributed Processing (1009).
Archived technical note: AtCoder. ABC 238 A Exponential or Quadratic.
Archived technical note: AtCoder. ABC 238 B Pizza.
Archived technical note: AtCoder. ABC 238 C digitnum.
Archived technical note: BOJ. Hue Circle (2482).
Archived technical note: BOJ. ACM Craft (1005).
Archived technical note: BOJ. Advertisement (1305).
Archived technical note: BOJ. Independent Set of Tree. (2213).
Archived technical note: BOJ. excellence town (1949).
Archived technical note: BOJ. Society Service(SNS) (2533).
Archived technical note: BOJ. Phone Keyboard (5670).
Archived technical note: BOJ. Clock pictures (10266).
Archived technical note: AtCoder. ABC 237 E Skiing.
Archived technical note: AtCoder. ABC 237 D LR insertion.
Archived technical note: AtCoder. ABC 237 C kasaka.
Archived technical note: AtCoder. ABC 237 B Matrix Transposition.
Archived technical note: AtCoder. ABC 237 A Not Overflow.
Archived technical note: BOJ. Find (1786).
Archived technical note: BOJ. String set (14425).
Archived technical note: BOJ. Ant tunnel (14725).
Archived technical note: BOJ. String square (4354).
Archived technical note: BOJ. Two Liquid (2470).
Archived technical note: BOJ. Knapsack problem (1450).
Archived technical note: BOJ. Sum of Two Numbers (3273).
Archived technical note: BOJ. Subsequence sum (1806).
Archived technical note: BOJ. Stick (1094).
Archived technical note: BOJ. Determine task (1311).
Archived technical note: BOJ. Line up (2252).
Archived technical note: BOJ. Matrix multiplication order (11049).
Archived technical note: BOJ. Consecutive prime sum (1644).
Archived technical note: BOJ. Set (11723).
Archived technical note: BOJ. Strongly Connected Component (2150).
Archived technical note: BOJ. Tree And Query (15681).
Archived technical note: BOJ. Make Bridge(2) (17472).
Archived technical note: BOJ. Planet Tunnel (2887).
Archived technical note: BOJ. MST (Minimum Spanning Tree) (1197).
Archived technical note: BOJ. Digital Video Disk (9345).
Archived technical note: BOJ. Josephus problem(2) (1168).
Archived technical note: BOJ. Data Structure (12899).
Archived technical note: BOJ. Police Car (2618).
Archived technical note: BOJ. Array and Query (16975).
Archived technical note: AtCoder. ABC 235 D - Multiply and Rotate.
Archived technical note: AtCoder. ABC 235 A - Rotate.
Archived technical note: AtCoder. ABC 235 B Climbing Takahashi.
Archived technical note: AtCoder. ABC 235 C The Kth Time Query.
Archived technical note: BOJ. Floyd(2) (11780).
Archived technical note: BOJ. Get Miminum Cost(2) (11779).
Archived technical note: BOJ. DSLR (9019).
Archived technical note: BOJ. Hide And Sick(4) (13913).
Archived technical note: BOJ. LCS 2 (9252).
Archived technical note: BOJ. Longest Increasing Subsequence(5) (14003).
Archived technical note: BOJ. Longest Increasing Subsequence(4) (14002).
Archived technical note: BOJ. Make into 1(2) (12852).
Archived technical note: BOJ. Iterate Tree (2263).
Archived technical note: BOJ. Bubble Sort (1517).
Archived technical note: BOJ. Tree Iteration (1991).
Archived technical note: BOJ. Diameter of Tree (1967).
Archived technical note: BOJ. Diameter of Tree (1167).
Archived technical note: BOJ. Find Parent In Tree (11725).
Archived technical note: BOJ. Maximum and Minimum Values (2357).
Archived technical note: BOJ. Binary Search Tree (5639).
Archived technical note: BOJ. Cycle game (20040).
Archived technical note: LeetCode. 131. Palindrome Partitioning.
Archived technical note: BOJ. Friend Network (4195).
Archived technical note: BOJ. Let's go on a trip (1976).
Archived technical note: BOJ. Merge Files (11066).
Archived technical note: BOJ. Exercise (1956).
Archived technical note: BOJ. KCM Travel (10217).
Archived technical note: BOJ. Floyd (11404).
Archived technical note: LeetCode. 997. Find the Town Judge.
Archived technical note: BOJ. Time Machine (11657).
Archived technical note: BOJ. Unidentified destination (9370).
Archived technical note: LeetCode. 312. Burst Balloons.
Archived technical note: BOJ. Shortest path with specification (1504).
Archived technical note: BOJ. Prefix Product (11505).
Archived technical note: BOJ. Shortest Path (1753).
Archived technical note: Programmers. Network.
Archived technical note: BOJ. Bipartite Graph (1707).
Archived technical note: BOJ. Move the Knight (7562).
Archived technical note: LeetCode. 40. Combination Sum II.
Archived technical note: AtCoder. 020 Log Inequality(3).
Archived technical note: AtCoder. 018 Statue of Chokudai(3).
Archived technical note: AtCoder. 014 We Used to Sing a Song Together(3).
Archived technical note: AtCoder. 015 Don't be too close(6).
Archived technical note: AtCoder. 016 Minimum Coins(3).
Archived technical note: AtCoder. 013 Passing(5).
Archived technical note: AtCoder. 009 Three Point Angle(6).
Archived technical note: AtCoder. 010 Score Sum Queries(2).
Archived technical note: AtCoder. 011 Gravy Jobs(6).
Archived technical note: AtCoder. 012 Red Painting(4).
Archived technical note: AtCoder. 008 AtCounter(4).
Archived technical note: AtCoder. 007 CP Classes(3).
Archived technical note: AtCoder. 006 Smallest Subsequence(5).
Archived technical note: AtCoder. 005 Restricted Digits(7).
Archived technical note: AtCoder. 004 Cross Sum(2).
Archived technical note: AtCoder. 001 Yokan Party(4).
Archived technical note: AtCoder. 002 Encyclopedia of Parentheses(3).
Archived technical note: AtCoder. 003 Longest Circular Road(4).
Archived technical note: BOJ. Expression of Set (1717).
Archived technical note: LeetCode. 39. Combination Sum.
Archived technical note: BOJ. Move and break wall (2206).
Archived technical note: BOJ. Prefix sum (2042).
Archived technical note: BOJ. Hide and Seek (1697).
Archived technical note: Programmers. Find number of animals with same name.
Archived technical note: Programmers — Largest Number.
Archived technical note: LeetCode. 38. Count and Say.
Archived technical note: BOJ. Tomato (7569).
Archived technical note: LeetCode. 37. Sudoku Solver.
Archived technical note: LeetCode. 36. Valid Sudoku.
Archived technical note: BOJ. Tell Median Value (1655).
Archived technical note: LeetCode. 33. Search in Rotated Sorted Array.
Archived technical note: LeetCode. 34. Find First and Last Position of Element in Sorted Array.
Archived technical note: LeetCode. 32. Longest Valid Parentheses.
Archived technical note: BOJ. Maximum heap (11279).
Archived technical note: Codility. ArrayInversionCount.
Archived technical note: LeetCode. 35. Search Insert Position.
Archived technical note: LeetCode. 31. Next Permutation.
Archived technical note: LeetCode. 30. Substring with Concatenation of All Words.
Archived technical note: LeetCode. 29. Divide Two Integers.
Archived technical note: Programmers. Stock Price.
Archived technical note: Programmers. Disk Controller.
Archived technical note: BOJ. Closest two Point (2261).
Archived technical note: LeetCode. 28. Implement strStr().
Archived technical note: LeetCode. 27. Remove Element.
Archived technical note: LeetCode. 26. Remove Duplicates from Sorted Array.
Archived technical note: BOJ. Bigest square in histogram (6549).
Archived technical note: LeetCode. 25. Reverse Nodes in k-Group.
Archived technical note: LeetCode. 24. Swap Nodes in Pairs.
Archived technical note: LeetCode. 23. Merge k Sorted Lists.
Archived technical note: LeetCode. 21. Merge Two Sorted Lists.
Archived technical note: LeetCode. 10. Regular Expression Matching.
Archived technical note: LeetCode. 17. Letter Combinations of a Phone Number.
Archived technical note: LeetCode. 20. Valid Parentheses.
Archived technical note: AtCoder Typical 90 021 — Come Back in One Piece.
Understand strongly connected components and Tarjan’s index, low-link, and active-stack invariants, with a worked graph and implementation.
Archived technical note: LeetCode. 19. Remove Nth Node From End of List.
De Moivre's formula for positive, zero, and negative integer powers, proved with Euler's formula and illustrated by a cube calculation.
Archived technical note: LeetCode. 1. Two Sum.
Archived technical note: LeetCode. 461. Hamming Distance.
Archived technical note: LeetCode. 540. Single Element in a Sorted Array.
Archived technical note: LeetCode 94. Binary Tree Inorder Traversal.
Archived technical note: BOJ 11659 — Range Sum Query 4.
Archived technical note: LeetCode. 53. Maximum Subarray.
Archived technical note: LeetCode. 13. Roman to Integer.
Archived technical note: LeetCode. 83. Remove Duplicates from Sorted List.
Archived technical note: AtCoder. 019 Pick Two(6).
Archived technical note: Contact.
Archived technical note: LeetCode. 14. Longest Common Prefix.
Archived technical note: LeetCode. 11. Container With Most Water.
Archived technical note: LeetCode. 12.Integer to Roman.
Archived technical note: LeetCode. 15. 3Sum.
Archived technical note: LeetCode. 16. 3Sum Closest.
Archived technical note: LeetCode. 3. Longest Substring Without Repeating Characters.
Archived technical note: LeetCode. 4 Median of Two Sorted Arrays.
Archived technical note: LeetCode. 5. Longest Palindromic Substring.
Archived technical note: LeetCode. 6. ZigZag Conversion.
Archived technical note: LeetCode. 7. Reverse Integer.
Archived technical note: LeetCode. 9. Palindrome Number.
A stable least-significant-digit radix sort for nonnegative safe integers, with input validation, complexity, and edge-case behavior.
Archived technical note: BOJ. Tree (4803).