brain

We are given a binary string consisting only of the characters a and b. For each test case, we must find any contiguous segment whose number of a characters is exactly equal to its number of b characters.

The problem involves a dynamometer sliding along a smooth horizontal table, pulled by a constant horizontal force $F=4~\text{N}$, with the mass of its spring equal to the mass of its casing.

We are given a group of players where every pair plays a single chess game. For each match, we must decide one of three outcomes: one player wins and the other loses, or both draw.

A passenger of mass $m$ moves along a curved path of radius $R$ with speed $v$ during a turn.

The problem is not a small implementation bug. The entire algorithmic approach is fundamentally wrong. Let's trace the sample that immediately disproves it: The proposed algorithm sees at least one '1', then only checks the first and last characters: - s[0] = ')' → flip needed…

A cubic vessel of edge length $a = 1~\text{cm} = 10^{-2}~\text{m}$ contains $n$ gas molecules at room temperature.

The physical system is a television image transmission channel.

We are given a rectangular game level defined by its width a and height b. Inside the level are n enemies, each at a unique position (xi, yi).

We are given several test cases. Each test case describes a set of doctors, where each doctor is available only during a fixed integer time interval.

We are given a string of brackets s and a binary string a of length n-3. Each '1' in a imposes a constraint that the corresponding 4-character substring of s must be a valid regular bracket sequence.

We are asked to place a collection of “events,” where each event is not a single day but a whole arithmetic progression of days.

Codeforces 1571G: A Battle Against a Dragon

A person jumps vertically upward from the surface of the Moon.

We are given several pairs of strings, each marked either as needing to rhyme or explicitly not rhyming. Two strings rhyme under a positive integer $k$ if their last $k$ characters are identical and each string has length at least $k$.

We are given a competition with n programmers and a survey of m spectators, each predicting who will finish first and last. You, as the first spectator, have submitted your prediction (f1, l1).

A perfectly conducting isolated metal sphere of radius $R$ is placed in a uniform external electric field of magnitude $E$ directed along a fixed axis.

We are given a sequence of relational signs between consecutive elements of an array, and we are asked to determine whether we can uniquely infer the relation between the first and the last element.

We have a one dimensional image represented by an array of colors. A single operation chooses a connected monochromatic segment and repaints the entire segment to any color. The connectivity definition is slightly unusual.

We have a row of cities, each of which can host a broadcasting station. Each station has a height and a range. A station can reach cities to its right up to its range limit, but it is blocked by any taller station in between.

We are given a universe of $2^n$ players, where each player is identified by an $n$-bit mask. Each bit represents whether that player has a positive or negative opinion on a topic.

A stretched string performs small transverse vibrations.

The system consists of a rigid tool, such as an awl, nail, or knife, interacting with a deformable material.

A submarine moves vertically downward with constant speed $u$ in still water where sound propagates with speed $V$.

We are given a sequence of 0s and 1s and an operation that can change any three consecutive elements to the XOR of those three elements.

The problem presents a digital clock showing an integer with exactly $n$ digits. Each digit ranges from 0 to 9, and the display includes leading zeroes so the number always has length $n$. The goal is to reduce this number to zero with the fewest possible operations.

An ideal gas containing $n$ moles undergoes a quasistatic thermodynamic process in which the temperature $T$ is prescribed as a function of the volume $V$.

Two identical solid elastic spheres of radius $R = 1~\text{cm} = 1.

We are given two arrays of length $n$, where array $a$ contains all the odd numbers from $1$ to $2n$ in some order, and array $b$ contains all the even numbers from $1$ to $2n$ in some order.

A horizontal cylinder contains $n=1$ mole of an ideal gas.

We have a squad of heroes, each with a fixed strength. For every dragon, we must choose exactly one hero to fight it, while all remaining heroes stay behind to defend the castle. A dragon has two requirements.

We are asked to construct a length m array a, where each position can take a value from 1 to k. The twist is that we are given several “pattern arrays” Ai, and these patterns impose constraints on how often they are allowed to appear inside a.

We are asked to fill an initially empty $n times m$ matrix with 0s and 1s such that every $2 times 2$ submatrix contains exactly two 0s and two 1s.

In this problem, we are asked to help Ivan choose the strongest possible hero build from a set of equipment slots, each with multiple items of different strengths. Each slot has its own list of items, already sorted by increasing strength.

The proposed solution appears to be a complete and well-explained editorial for Codeforces 1575A, but it does not address Exercise 4.2.2.16 from TAOCP at all.

A solid sphere of radius $R$ carries a uniform volume charge density $\sigma$ with units $\mathrm{C/m^3}$.

A rigid conducting rod of length $l = 0.

We are asked to generate valid bracket sequences, which are strings consisting only of "(" and ")" that form balanced parentheses. A sequence is valid if every opening bracket has a corresponding closing bracket and no closing bracket appears before its matching opening bracket.

We have a collection of distinct book titles, all with the same length. The books are not sorted using ordinary lexicographic order. When comparing two titles, we scan from left to right until we find the first position where they differ.

We start with an array. We may repeatedly delete arbitrary elements, and after each deletion the remaining elements close up together. For any resulting array, define its score as the number of positions where the value equals its current 1-based index.

The grid describes a city map where some cells contain telephone poles. Each cell corresponds to an integer coordinate point on a plane, and a value of 1 means a pole exists at that location.

The system consists of two identical point masses, each of mass $m$, fixed at the ends of a massless rigid rod of length $l$.

We are asked to count the number of ways to color a large rectangular cloth with size $n times m$ using $k$ colors, given that two fixed subrectangles of size $r times c$ must be identical in their color patterns.

A homogeneous plasma initially occupies a slab of thickness $x$ and contains a uniform density $n$ of positive ions and an equal density $n$ of electrons, where $n$ is the number of particles of each…

We have an n × m grid. Every cell stores one of three directions. A value of 1 means a ball moves one cell to the right. A value of 2 means the ball moves one cell downward. A value of 3 means the ball moves one cell to the left.

We are given an array of integers a with length n. The goal is to compute a sum over all pairs (i, j) where i and j are indices of the array, and for each pair we multiply two quantities: the GCD of the array elements a[i] and a[j], and the GCD of their positions i and j.

We are given a binary string representing a wall, where each character is either 0 or 1. This string, a, is of length n. Mr. Chanek wants to place his nephew’s favorite pattern, another binary string b of length m, onto the wall.

We start with an array whose values lie in the range [0, k - 1]. As long as not all positions contain the same value, we repeatedly choose a random position and a random value, then overwrite that position.

The city park is a tree where each attraction is a node and each rail is an edge. Every node has a happiness value, and every edge has a color, either black or white. Moving through the tree is always along simple paths, meaning you never revisit a node.

We want to place a circular amusement park so that it touches the origin. If the park has radius $r$, then its center must lie exactly $r$ units from the origin, because the origin lies on the boundary of the park. Each bird habitat is a point in the plane.

The physical system is a layer of fog consisting of identical spherical water droplets suspended in air.

We are given a very short string, at most length 8, that represents a partially unknown integer. Some positions contain fixed digits, some contain a wildcard underscore meaning “any digit is allowed here”, and some contain the character X meaning all X positions must share…

We are given an array a of length n and a repetition count m. From this, we form a cyclic sequence b by concatenating m copies of a. Conceptually, b is circular: after the last element, the first element follows.

The system models a communication network where each node represents a switching site and each edge represents a physical communication link between two sites. Every link has a limited capacity, a length cost, and belongs to a group.

The radiation of a fixed infrared wavelength propagates through methane according to the exponential attenuation law.

We are working in a plane where a circle is hidden from us. The circle is fully determined by its center coordinates and its radius, but we do not know them. Our only tool is to shoot a ray starting from the origin and passing through an integer point we choose.

A football is launched from a fixed point on level ground with initial speed $v$ at an angle $\alpha=30^\circ$ to the horizontal.

We are given a single string that represents a type expression built from three constructs: a base unit type written as (), parentheses that group a type without changing its meaning, and a function constructor written as - that connects two types.

We are asked to schedule tasks that are arranged as a full binary tree of height $h$. A full binary tree of height $h$ has $2^h - 1$ nodes. Initially, only the root task is ready. At each discrete moment of time, up to $p$ processes can perform ready tasks simultaneously.

We are given a cactus graph, which is a connected undirected graph where each vertex lies on at most one simple cycle. That means every vertex is either part of a tree structure or part of a single cycle. No multi-edges or loops are allowed.

Every unit square with integer corners is crossed by exactly one segment of one of four infinite dragon curves. For a query square with opposite corners $(x,y)$ and $(x+1,y+1)$, we must determine two things.

We are given an array consisting only of zeroes and ones, and a positive integer d that specifies a cyclic right shift.

The proposed solution does address the actual exercise and identifies the key idea: each pair $(U{2n-1},U{2n})$ is transformed by either keeping it unchanged or swapping coordinates, depending on whether the point lies in $G$. This is indeed the mechanism behind the construction.

We are given a sequence of segment lengths, and we must place these segments one after another on an infinite number line. The first segment starts at coordinate 0, with one endpoint fixed at 0.

We are given a permutation of size $n$, which means an array containing all integers from 1 to $n$ exactly once. Our task is to process this permutation into a double-ended queue, or deque, where each new element can either be placed at the front or the back.

We are given a grid of size $n times m$ representing a paper, where some cells are black (denoted by ) and others are white (.). Casimir claims the black cells are produced by drawing ticks.

We are given a string containing only the characters A, B, and C. In one move, we may remove either an A together with a B, or a B together with a C. The two removed characters can come from any positions in the string.

We are asked to organize a meeting with $n$ people, where each person has a limit on how many private conversations they can participate in. The array $a$ describes each person’s sociability: $ai$ is the number of talks person $i$ can attend before leaving.

We are given an array and a special operation. Instead of swapping individual elements, we may choose any contiguous segment and rotate that segment to the left by any amount. After the rotation, the elements remain inside the same interval, only their order changes cyclically.

We are given a network of railway stations in Gensokyo, connected by two-way railways with positive lengths. The stations are numbered from 1 to $n$, with station 1 being the main hub. The network is fully connected, so every station is reachable from any other station.

We have a system that tracks trains of different models over a sequence of days. At the start, there are no trains. On each day, we either add a train of a specific model or remove one.

An ideal gas in equilibrium is described by states $(p,V)$ with $p>0$, $V>0$ and equation of state $pV=\nu RT$.

I can write a full editorial in that format, but I need the actual solution first. Codeforces 1580F is a 3300-rated problem whose accepted solution is highly nontrivial and relies on fairly deep generating-function techniques with FFT.

We are given a sequence of distinct integers of length $n$, and we are asked to select a subsequence of length $m$ to maximize a specific value.

We are given a binary grid. A cell containing 1 is an obsidian block and a cell containing 0 is empty. We may flip any cell, changing 0 to 1 or 1 to 0, with cost one per flip.

We are asked to count permutations of the numbers from 1 to $n$ that have exactly $k$ "good" numbers. A number $x$ in a permutation is called good if, when we look at all contiguous subarrays containing $x$, there are exactly $m$ distinct maxima.

The outputs: show that the current algorithm is not merely suffering from a small implementation bug. It is solving a different optimization problem. Let's examine the samples: Input: Expected: Current output: 15 is simply the sum of the array.

We are asked to determine whether it is possible to construct a simple, connected, undirected graph with a given number of nodes n and edges m such that the graph's diameter is strictly less than k-1.

We are given an array of integers, and the task is to determine all possible values of x that can be obtained as the XOR of some increasing subsequence of the array.

A rectangular block of height $H$ and base dimensions $a \times b$ stands on a horizontal rigid surface.

The solution correctly identifies that each ball undergoes uniform circular motion about its respective pivot, with angular velocities $\omega_1 = v/l$ and $\omega_2 = 2v/(2l) = v/l$, so the angular s…

We are given an array of integers a of length n. The task is to find all non-negative integers x that can be expressed as the XOR of some strictly increasing subsequence of a.

Consider a straight line segment $AB$ carrying a uniform linear charge density $\lambda$.

The physical system consists of a rigid container containing an ideal gas whose bulk temperature is $T_1$, while the walls are maintained at temperature $T$.

We are given several arrays. For each array, let the sum of all elements be $s$. We need to count how many subsequences have sum exactly $s-1$. A subsequence is formed by deleting any number of elements, possibly none of them or all of them.

The fragment “NOT_FOUN_” contains no equations, constraints, or target quantity, so the problem cannot be instantiated as a well-posed Kvant task.

A flag is modeled as a thin flexible sheet of characteristic length $L$ and mass per unit area $\sigma$, clamped along one edge to a rigid pole.

The physical system consists of a person moving on an icy slope inclined at an angle $\alpha$ to the horizontal.

We are given a rectangular board where each cell is either blocked by an X or empty. From an empty cell we may move only upward or leftward. A cell is called exitable if some sequence of such moves lets us leave the grid. The question is not to simulate those moves.

The system consists of two conducting plates and the ground.

The system consists of a thin metallic ring of mass $m$ and resistance $R$, with diameter $d$, falling vertically in a magnetic field that points along the vertical direction.

We are given up to 10 strings, all over a mixed alphabet of lowercase and uppercase English letters. The task is to construct a single string that appears as a subsequence in every given string, and among all such strings we want the longest possible one.

We have several points on the plane. We want to count pairs of points whose connecting segment stays close to every point in the set. More precisely, for a chosen pair $(pi,pj)$, every point must lie at distance at most $R$ from that segment.

For each test case we are given two positive integers, $u$ and $v$. We must find any pair of integers $x$ and $y$, not both zero, such that $$frac{x}{u}+frac{y}{v}=frac{x+y}{u+v}.$$ The task is not to count solutions or find a particular one.

Two identical balls, each of mass $m$, are connected by a massless spring of stiffness $k$.

We are given a sequence of piles of stones, each with a non-negative integer count. Bob can repeatedly remove one stone from two adjacent piles. If a pile becomes empty, it no longer counts as part of any adjacent pair.

We are given a sequence of length $n$ that initially contains integers from $1$ to $n$ in increasing order. Then the jury selects three indices $i < j < k$ with $j - i 1$ and reverses two subsegments: the first from $i$ to $j-1$, and the second from $j$ to $k$.

We start with an $n times m$ grid of cells. The rectangle may be cut repeatedly along grid lines, producing smaller rectangles. The only restriction is that no final piece may have size $1 times 1$.

We are given two integer arrays of equal length. The allowed operation on the first array is quite specific: we may choose some subset of positions and increase each chosen element by exactly one. After that, we are allowed to freely reorder the array.

An arrow of mass

A camera obscura is modeled as a rectangular box of length $L$ with a small circular aperture of diameter $d = 1,\text{mm} = 1\times 10^{-3},\text{m}$.