brain

tamnd's digital brain — notes, problems, research

41641 notes

CF 104724A - lock

We are dealing with a circular lock made of five digits, each digit ranging from 0 to 9, where incrementing past 9 wraps back to 0.

codeforcescompetitive-programming
CF 104733D2 - Win as Second D2

The problem statement is missing from your message, so I can’t produce a correct editorial yet. Codeforces “D2” tasks in particular usually depend heavily on precise rules, constraints, and sometimes interactive or constructive conditions.

codeforcescompetitive-programming
CF 104733C1 - Mascot Maze C1

We are given a changing maze of chambers connected by corridors that appear over time and then disappear after a fixed duration. People start in a small set of starting chambers, while exits are located in the last few chambers.

codeforcescompetitive-programming
CF 104733D1 - Win as Second D1

I’m missing the actual problem statement for Codeforces 104733D1 - Win as Second D1, so I can’t reliably reconstruct the task, constraints, or required technique.

codeforcescompetitive-programming
CF 104733C2 - Mascot Maze C2

I don’t have the actual statement of Codeforces 104733C2 - Mascot Maze C2 in the prompt you provided (the problem body is missing after “Problem Statement”).

codeforcescompetitive-programming
CF 104733B2 - Duck, Duck, Geese B2

I’m missing the actual problem statement for Codeforces 104733B2 - Duck, Duck, Geese B2 (the “Input” and “Output” sections are empty in what you provided).

codeforcescompetitive-programming
CF 104733A2 - Revenge of GoroSort A2

I can’t reliably reconstruct “Revenge of GoroSort A2” from the Codeforces gym link alone, and I don’t want to hallucinate an editorial for the wrong problem.

codeforcescompetitive-programming
CF 104733B1 - Duck, Duck, Geese B1

I can write the full editorial in the exact style you requested, but the problem statement is missing from your prompt.

codeforcescompetitive-programming
CF 104733A3 - Revenge of GoroSort A3

I can’t write a correct editorial for this yet because the actual problem statement is missing. Right now I only have the title “104733A3 - Revenge of GoroSort A3”, but no description of the task, inputs, or outputs.

codeforcescompetitive-programming
CF 104730I - Выиграй МКОШП

We are given a small team of up to 12 students and up to 100 monsters. Each student has three attributes: current health, attack power, and a one-time shielding ability that can be used to increase any student’s health.

codeforcescompetitive-programming
CF 104730J - Путёвка на Острова Кука

We are given several independent test cases. In each test case, there are $3n$ points on the plane, all with integer coordinates and all distinct.

codeforcescompetitive-programming
CF 104730H - Задача в подарок

We are given a collection of problems, each with a non-negative difficulty value, and a total mental budget $S$. We may choose a subset of problems whose total difficulty does not exceed $S$, and these are considered “solved normally”.

codeforcescompetitive-programming
CF 104730G - Good Colorings

We are given an $n times n$ grid. At the start, Alice has already colored exactly $2n$ distinct cells, and each of these cells is assigned a unique color from $1$ to $2n$.

codeforcescompetitive-programming
CF 104730F - Split

We are given a permutation of size $n$, meaning every value from $1$ to $n$ appears exactly once in the array. For each query, we look at a contiguous segment and ask whether it can be split into two consecutive parts such that every value in the left part is strictly smaller…

codeforcescompetitive-programming
CF 104730E - Time Travel

We are given a fixed set of cities, but the road network between them changes over time. Each “time moment” describes a different undirected graph on the same set of cities, and there are up to 200000 such snapshots. You are also given a fixed sequence of time jumps.

codeforcescompetitive-programming
CF 104730B - Игра джентльменов

We are given a collection of cards, each card contains an array of length n. There are n players, and exactly m cards available. The players take turns in a fixed order from player 1 to player n, and each player picks exactly one card from those still available.

codeforcescompetitive-programming
CF 104730D - Minimum Segments

We are given a sequence $r1, r2, ldots, rn$. This sequence does not come from the original array directly, but from a derived process applied to some hidden array $a$, where each $ai$ is an integer between 1 and $n$.

codeforcescompetitive-programming
CF 104730C - Minimum Array

We start with an initial array and a sequence of range updates that are applied one after another. After each prefix of these operations, we obtain a new version of the array.

codeforcescompetitive-programming
CF 104728M - 近似递增序列

We are counting sequences of positive integers where the product of all elements is at most a given limit, and the sequence is “almost increasing” in the sense that along the sequence there is at most one position where the monotonic increase condition fails.

codeforcescompetitive-programming
CF 104728L - Azur Lane

We are given the final state of a sequence of loot boxes after several days of operations. Each day, some multiset of boxes was obtained, then internally sorted in non-increasing order of rarity, and appended to the existing sequence.

codeforcescompetitive-programming
CF 104728E - 序列配对

We are given a sequence of length $n$, initially all zeros. Alongside this sequence comes a list of $n$ pairing operations, each operation connects two indices $l$ and $r$.

codeforcescompetitive-programming
CF 104728J - 基因编辑

We are given a collection of DNA strings, each over the alphabet {A, C, G, T}. From any ordered pair of strings, we are allowed to form a new string by taking a prefix of the first string and concatenating it with a suffix of the second string.

codeforcescompetitive-programming
CF 104728H - 狭义线段树

We are given a fixed rooted binary tree with $2n-1$ nodes and $n$ leaves. The nodes are labeled in DFS order, so subtree intervals correspond to contiguous segments of this labeling.

codeforcescompetitive-programming
CF 104728G - 广义线段树

We are given a rooted binary tree with exactly $2n-1$ nodes. The leaves correspond one-to-one with positions of an array $a$, and every internal node represents a contiguous interval formed by merging its left and right children.

codeforcescompetitive-programming
CF 104728F - 新取模运算

We are given a prime number $p$, and many queries. Each query provides a huge integer $n$, and we need to evaluate a custom operation on $n!$.

codeforcescompetitive-programming
CF 104728B - 逆 KMP

We are given a length-n array of constraints. For each position i, a value a[i] tells us that the first a[i] characters of the final sequence must match a block of length a[i] ending at position i. In other words, for every i, the segment s[1..

codeforcescompetitive-programming
CF 104728D - 网格染色

We are given an $n times n$ grid of unit squares. The grid edges are initially uncolored. Two players alternate turns, with Walk Alone starting first. On each move, a player chooses any currently uncolored edge and colors it in their own color.

codeforcescompetitive-programming
CF 104725B - 终焉之茧

We are playing an interactive search game on a 2D integer grid. There is a hidden target point with integer coordinates bounded inside a square around the origin, and we start from the origin.

codeforcescompetitive-programming
CF 104725G - 精灵宝可梦对战

We are simulating a turn-based duel between two ordered teams of Pokémon-like fighters. Each team is a queue of units, and at any moment only the front unit of each team is active. The two players alternate turns, starting with Alice.

codeforcescompetitive-programming
CF 104725K - RSP

We are given a two-player game that generalizes rock-paper-scissors to $n$ symbols arranged in a cycle. Symbol $i$ defeats symbol $i+1$, and symbol $n$ defeats symbol $1$. Any other pairing that is not a direct win relation results in a draw.

codeforcescompetitive-programming
CF 104725L - 养成游戏

We are given a small system of integer variables. There are up to six variables, each representing an attribute of a character in a game, and each attribute can be any integer from 0 to K. Alongside these variables, there are up to 100 judges.

codeforcescompetitive-programming
CF 104725I - 幸福前方的物语

We are given an interval of integers from $l$ to $r$. From this interval we consider all subsets, including the empty subset. For any subset, we multiply all chosen numbers and check whether the product is a perfect square.

codeforcescompetitive-programming
CF 104725J - 圣夜的奇迹跑者

We are given a one-dimensional race track represented by integer positions from 1 to m. A special “perfect zone” is the suffix interval [R, m], and the final goal is to maximize the chance that a specific skill is the k-th skill to successfully trigger inside this perfect…

codeforcescompetitive-programming
CF 104725H - 字符串游戏

We are given a collection of strings owned by one player, and a second collection of strings used to generate queries. For each query string, we consider every one of its substrings as a separate game instance.

codeforcescompetitive-programming
CF 104725F - 最长上升子序列

We are given an array over positions, where each position i comes with a number a[i]. This number is meant to represent the length of the longest strictly increasing subsequence that ends exactly at position i in some hidden permutation p of 1 to n.

codeforcescompetitive-programming
CF 104725E - 永世乐土

We are given an undirected graph with up to 30 nodes and 50 edges. Several special nodes are marked as “memory locations”, and each of these contains one or more memories of interest. We start at node 1 and can walk along edges step by step.

codeforcescompetitive-programming
CF 104725D - 金人旧巷市廛喧

The grid describes a city map where movement is only allowed through passable cells and only in four directions. Some cells are blocked, some cells provide a bonus, and all other cells are neutral. There are exactly $k$ starting positions and $k$ ending positions.

codeforcescompetitive-programming
CF 104725C - 国王的疑惑

We are given a large directed complete graph structure that is not meant to be processed explicitly. The full graph consists of $K$ identical blocks, each block containing $n$ cities.

codeforcescompetitive-programming
CF 104725A - 疾羽的救赎

We are simulating a small board game played on a linear track of nine cells. Initially, there are three distinct pieces placed on fixed positions: a purple piece starts at cell 2, a green piece at cell 3, and a yellow piece at cell 4.

codeforcescompetitive-programming
CF 104728N - A+B problem

Each input consists of two single uppercase letters, and each letter represents a number in base 26 where A corresponds to 0, B to 1, and so on up to Z as 25.

codeforcescompetitive-programming
CF 104728K - 不定项选择题

We are dealing with a situation where there are $n$ distinct options, and the correct answer is some unknown non-empty subset of these options. Initially, no option is selected.

codeforcescompetitive-programming
CF 104728I - Fujisaki 讨厌数学

We are given an integer relation that behaves like a hidden exponential variable. There exists some number $x$ (not necessarily integer) such that its value together with its reciprocal satisfies $x + frac{1}{x} = k$, where $k$ is a fixed integer at least 2.

codeforcescompetitive-programming
CF 104728A - 简单的加法乘法计算题

We start with a value x = 0 and want to reach a target value y. We are allowed two types of operations. The first type adds any integer from 1 to n to the current value. The second type multiplies the current value by one of up to m given multipliers.

codeforcescompetitive-programming
CF 104728C - 排列排序问题

We are given a permutation of the numbers from 1 to n, and we want to transform it into the sorted sequence 1, 2, 3, ..., n using a special kind of restructuring operation.

codeforcescompetitive-programming
CF 104730A - Уникальная песня

We are given two collections of strings, each of size $n$. We must arrange them into a single sequence of length $2n$, but the positions are fixed by parity: every odd position must contain a string from the first collection, and every even position must contain a string from…

codeforcescompetitive-programming
CF 104733A1 - Revenge of GoroSort A1

We are given a line of boxes, each initially containing exactly one labeled ball. The labels form a permutation-like arrangement, so each box holds one distinct ball number. The goal is to transform this arrangement into the sorted state where ball i ends up in box i.

codeforcescompetitive-programming
CF 104735F2 - Go To Considered Helpful F2

I can write the full Codeforces-style editorial, but I’m missing the actual problem statement for 104735F2 - Go To Considered Helpful F2 in your prompt.

codeforcescompetitive-programming
CF 104735F1 - Go To Considered Helpful F1

The problem statement for Codeforces 104735F1 - Go To Considered Helpful F1 is missing from your prompt, so I can’t reconstruct the logic, constraints, or solution path reliably.

codeforcescompetitive-programming
CF 104735B2 - Sorting Permutation Unit B2

We are given a permutation of numbers from 1 to n, placed on positions 1 through n. We are allowed to rearrange it using a very specific move: we can only swap elements whose indices differ by exactly k.

codeforcescompetitive-programming
CF 104735E2 - Juggle Struggle: Part 2 E2

I can’t write a correct editorial yet because the problem content for Codeforces 104735E2 - Juggle Struggle: Part 2 E2 is missing from your prompt. Right now I only see the title and format, but no statement, constraints, input/output format, or samples.

codeforcescompetitive-programming
CF 104735E1 - Juggle Struggle: Part 2 E1

The problem statement is missing from your prompt, so I don’t have enough information to write a correct editorial. Right now I only see the title “Codeforces 104735E1 - Juggle Struggle: Part 2 E1”, but no description, input/output format, or constraints.

codeforcescompetitive-programming
CF 104735D2 - Juggle Struggle: Part 1 D2

I can’t write a correct editorial yet because the actual problem statement for Codeforces 104735D2 - Juggle Struggle: Part 1 D2 is missing from your prompt.

codeforcescompetitive-programming
CF 104735D1 - Juggle Struggle: Part 1 D1

I don’t have the actual statement of Codeforces 104735D1 - Juggle Struggle: Part 1 D1 in your prompt, so I can’t safely reconstruct the problem or write a correct editorial.

codeforcescompetitive-programming
CF 104735C2 - Won't Sum? Must Now C2

I can’t complete the editorial for Codeforces 104735C2 - “Won’t Sum? Must Now C2” because the actual problem statement is not available in your prompt, and it is not uniquely recoverable from the title alone.

codeforcescompetitive-programming
CF 104735C1 - Won't Sum? Must Now C1

I’m missing the actual problem statement for Codeforces 104735C1 - Won’t Sum? Must Now C1, and the input/output details are empty in your message.

codeforcescompetitive-programming
CF 104735A2 - Board Meeting A2

I can’t write a correct editorial for this yet because the problem statement for “Codeforces 104735A2 - Board Meeting A2” isn’t included in your prompt.

codeforcescompetitive-programming
CF 104735A1 - Board Meeting A1

I can’t reliably reconstruct the intended solution yet because the full statement for Codeforces 104735A1 - Board Meeting A1 is not available in your prompt, and this problem ID does not correspond to a commonly indexed public statement in the usual Codeforces archive…

codeforcescompetitive-programming
CF 104735B1 - Sorting Permutation Unit B1

I can’t reliably write a correct, problem-specific editorial yet because the actual statement for Codeforces 104735B1 - Sorting Permutation Unit B1 is missing from your prompt.

codeforcescompetitive-programming
CF 104767C - Digitalisation

Each student arrives with two ranked preferences over schools. Students are already sorted by a global quality order, so we always process higher scoring students first.

codeforcescompetitive-programming
CF 104767G - Hamster

We are given a set of unit segments drawn on the integer grid. Each segment connects two grid intersection points that are exactly one step apart horizontally or vertically.

codeforcescompetitive-programming
CF 104761I - Игра на дереве

We are given a tree with $n$ vertices. One player first selects a vertex $u$, then an adversary selects a different vertex $v$. After that, a vertex $w$ is chosen uniformly at random from all $n$ vertices.

codeforcescompetitive-programming
CF 104761H - Равномерное кодирование

We are asked to design a binary codebook for a given number of commands $K$. Each command is a binary string of some even length $Len$, and we must assign $K$ distinct strings. These strings are not arbitrary. They must satisfy two structural constraints.

codeforcescompetitive-programming
CF 104761F - Справедливый разрез

We are given a fixed triangle placed in a coordinate system. One vertex is at the origin, a second vertex is at $(a,b)$, and the third is on the x-axis at $(c,0)$. Inside this triangle, a point $P$ is already fixed and guaranteed to lie on one of its sides.

codeforcescompetitive-programming
CF 104761G - Найти слона

We are interacting with a hidden chess problem on an 8 by 8 board where a bishop is placed on an unknown square. We do not know its position, but we can query any square and receive feedback about the minimum number of bishop moves required to reach that square from the hidden…

codeforcescompetitive-programming
CF 104761E - Цифровизация

Each dataset describes a single livestock breed. Inside a breed we are given many records, and each record corresponds to one animal passport. A passport contains three identifiers: the calf itself, its father, and its mother.

codeforcescompetitive-programming
CF 104761B - Занавеска

We are simulating a deterministic process that gradually “fills” positions from a line segment of length $N$. The positions are numbered from $1$ to $N$. The process starts by immediately selecting the two endpoints, so positions $1$ and $N$ are used at step 1.

codeforcescompetitive-programming
CF 104761D - Игра с бумагой

We start with a rectangular grid of size $W times H$. Each move allows us to cut the rectangle along a grid line, splitting it into two smaller rectangles either horizontally or vertically.

codeforcescompetitive-programming
CF 104761C - Делимость на 2023

We are given two distinct digits, call them $A$ and $B$, each between 0 and 9. From only these two digits, we are allowed to construct any positive integer by concatenating them in any order and any length, as long as we do not use any other digit.

codeforcescompetitive-programming
CF 104760I - Налоговый вопрос

We are given a graph of planets where each planet is connected to some others by two-way direct routes. For each planet, we care only about how many direct routes are incident to it, which is its degree in graph terms.

codeforcescompetitive-programming
CF 104760E - Old Cipher

We are given a sequence of natural numbers that represent encrypted keys. The task is to reorder these numbers according to a custom sorting rule that is not based on their usual numeric value, but on a transformation of each number.

codeforcescompetitive-programming
CF 104760D - Веселые фонари

The city can be modeled as an undirected graph where each lamp post is a vertex and each street is an edge between two vertices. The task is to decide whether we can assign one of two colors to each vertex so that every street connects vertices of different colors.

codeforcescompetitive-programming
CF 104760A - Много ракушек

We are given a linear sequence of shells, each with a positive weight. The shells are arranged in order along a beach, and we are only allowed to take a consecutive block of them.

codeforcescompetitive-programming
CF 104758C - Counting Decorations

We are building a triangular decoration with $N$ horizontal levels. Level $i$ contains exactly $i$ stickers, and every sticker must be colored using one of three colors: red, green, or blue.

codeforcescompetitive-programming
CF 104758A - Alaric Journey

We are given a sequence of integers arranged in a line. In one move, we may take two neighboring elements and replace them with their sum, effectively shortening the sequence by one element while preserving order elsewhere.

codeforcescompetitive-programming
TAOCP 7.2.2.1 Exercise 91

Fix a positive integer $W$.

taocpmathematicsalgorithmsvolume-4project
CF 104755F - Pinholes

We are working in a one-dimensional geometric setup where the vertical structure is fixed but the horizontal placement matters. There is a ceiling line at height 2, a floor at height 0, and a middle “screen” at height 1 that contains fixed holes at given x-coordinates.

codeforcescompetitive-programming
CF 104755B - Checkmate

We are given a single black king placed somewhere on an otherwise empty 8 by 8 chessboard. Our task is to place some white pieces, chosen from the standard set of chess pieces excluding quantity limits, so that the black king is checkmated.

codeforcescompetitive-programming
CF 104755L - Reconstruction

We are given a hidden convex polygon with $n$ vertices. Instead of being shown the polygon directly, we receive a multiset of geometric “snapshots”, where each snapshot is a triangle formed by choosing three vertices of the polygon and recording their coordinates.

codeforcescompetitive-programming
CF 104752I - Inspecting the Scores

Each game consists of a sequence of $N$ matches. The score starts at 1, and every match either multiplies the current score by $A$, multiplies it by $B$, or leaves it unchanged if nobody solves the problem. The key point is that we are not choosing a single sequence.

codeforcescompetitive-programming
CF 104752G - Game of Coin Stacking

We are given a sequence of coin stacks, each stack having some initial number of coins. Two players alternate turns and on each turn a player may pick any stack and remove any positive number of coins not exceeding what is currently in that stack.

codeforcescompetitive-programming
CF 104752A - Akira

We are given a set of points in the plane, each representing a source of an expanding circular “sphere of light”. Every point starts as a level 1 catastrophe, and its influence grows outward uniformly over time at a fixed rate $M$.

codeforcescompetitive-programming
CF 104752D - Determine Palindrome Message

We are given a single string consisting of lowercase Latin characters, but it may also contain digits or other characters in the input format examples, so we treat it as a sequence of symbols.

codeforcescompetitive-programming
CF 104745D - jbum

We are simulating a very simple production process that increases the number of discs Javier has over time. He starts with a single disc.

codeforcescompetitive-programming
CF 104745P - Ski resort

We are given a directed acyclic graph representing ski pistes, plus a small number of additional directed edges representing ski lifts. Every edge, whether piste or lift, takes exactly one minute to traverse.

codeforcescompetitive-programming
CF 104743B - Array Construction

We are asked to decide whether it is possible to build an array of length n using distinct integers such that two global bitwise constraints are satisfied simultaneously.

codeforcescompetitive-programming
CF 104743F - Yet Another Tree Problem

Working

codeforcescompetitive-programming
CF 104743C - Prefix MEX Problem

We are given an array of non-negative integers. We are allowed to modify elements, but each modification has a very specific rule: if we choose position i, we overwrite a[i] with the MEX of the prefix strictly before it, meaning the smallest non-negative integer that does not…

codeforcescompetitive-programming
CF 104743E - Range Modulo Queries

We are given two arrays of the same length. For every query interval, we are asked to find a modulus value $m$ such that when we take every element $ai$ in that interval and compute $ai bmod m$, we obtain exactly the corresponding $bi$.

codeforcescompetitive-programming
CF 104743A - Make All Elements 0

We are given an array of non-negative integers. In a single operation, we choose a contiguous segment and apply a bitwise AND with some value $x$, where $1 le x le k$. This operation overwrites every element in the chosen segment by clearing some of its bits according to $x$.

codeforcescompetitive-programming
CF 104741B - 小M的游戏

We are given a weighted undirected graph with $N$ locations and $M$ roads. Two players start at node $1$ and want to reach node $N$.

codeforcescompetitive-programming
CF 104741L - 兄弟校问题

We are given a collection of schools. Each school has a name and a city. We also have a list of keyword strings. A school is considered directly related to a keyword if that keyword appears as a whole token inside the school name, where tokens are the parts separated by…

codeforcescompetitive-programming
CF 104741C - 方格染色

We are given a grid shaped like two rows and n columns, so there are 2n cells arranged in a rectangle. Each cell must be colored either black or white.

codeforcescompetitive-programming
CF 104736L - Latam++

We are given a single string made of lowercase letters, parentheses, and arithmetic operators. Each substring of this string is interpreted as a potential expression in a very simple programming language.

codeforcescompetitive-programming
CF 104736D - Deciphering WordWhiz

We are given a fixed dictionary of five-letter words, where every word uses five distinct lowercase letters. The first word in this dictionary is the hidden target word for a single game session.

codeforcescompetitive-programming
CF 104736M - Meeting Point

We are given a weighted undirected graph representing a city where intersections are nodes and roads are edges with positive lengths. From a starting intersection $P$, we consider shortest-path distances as the true travel distances.

codeforcescompetitive-programming
CF 104736J - Journey of the Robber

We are given a tree with $N$ cities. Each city is identified by an integer from 1 to $N$, and this numbering is also its wealth rank: larger index means richer city.

codeforcescompetitive-programming
CF 104736K - Keen on Order

We are given a sequence of movie screenings over N days, where each day shows exactly one movie from a set of K distinct movies labeled from 1 to K.

codeforcescompetitive-programming
CF 104736G - GPS on a Flat Earth

We are given several radio towers on a 2D integer grid. Each tower knows its exact Manhattan distance to an unknown user position, and all these distances are guaranteed to be correct simultaneously.

codeforcescompetitive-programming
CF 104736I - Inversions

We are given a base string consisting of lowercase letters, and we conceptually build a much longer string by repeating this base string many times.

codeforcescompetitive-programming
CF 104736H - Health in Hazard

We are working in an infinite 2D plane where the origin is fixed at the point $(0,0)$. The bear can only consider points that lie exactly at Euclidean distance $D$ from the origin, so geometrically this is a circle centered at the origin.

codeforcescompetitive-programming