Questions tagged [combinatorics]

For challenges involving combinatorics.

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51 votes
2 answers
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Extending OEIS: Counting Diamond Tilings

I promise, this will be my last challenge about diamong tilings (for a while, anyway). On the bright side, this challenge doesn't have anything to do with ASCII art, and is not a code golf either, so ...
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50 votes
47 answers
4k views

Count sums of two squares

Given a non-negative number n, output the number of ways to express n as the sum of two squares of integers ...
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  • 138k
47 votes
8 answers
5k views

Determine if a coin system is Canonical

The Cashier's Algorithm is an algorithm for making change in the minimal number of coins that works quite well for most currency systems. However like most greedy algorithms it is not without its ...
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  • 89.4k
43 votes
21 answers
3k views

Has My Pie Been Bisected?

Write a program or function that takes in a nonempty list of positive integers. You may assume it is input in a reasonable convenient format such as "1 2 3 4" or <...
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42 votes
64 answers
9k views

Generate Pascal's triangle

Pascal's triangle is generated by starting with a 1 on the first row. On subsequent rows, the number is determined by the sum of the two numbers directly above it to the left and right. To ...
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  • 7,672
40 votes
45 answers
6k views

Every word from babab to zyzyz

Your task is to write a program that will output a readable list of every five letter words with the structure: consonant - vowel - consonant - vowel - consonant The output should be sorted ...
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40 votes
54 answers
7k views

Catalan Numbers

The Catalan numbers (OEIS) are a sequence of natural numbers often appearing in combinatorics. The nth Catalan number is the number of Dyck words (balanced strings of parenthesis or brackets such as ...
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40 votes
8 answers
3k views

Lazy battleship placement

Imagine the following scenario: you are playing battleships with a friend but decide to cheat. Rather than moving a ship after he shoots where your ship used to be, you decide not to place any ships ...
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  • 9,472
40 votes
0 answers
1k views

Topologically distinct ways of dissecting a square into rectangles

I was asked by OEIS contributor Andrew Howroyd to post a Code Golf Challenge to extend OEIS sequence A049021. Would be super great to get a couple more terms for [...] A049021. Kind of thing [...] ...
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  • 8,117
39 votes
10 answers
5k views

The Urinal Protocol

Background The so-called "Urinal Protocol", describing the order in which individual urinals are picked in a men's bathroom, has been discussed in multiple places. One version is given in this xkcd ...
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  • 2,919
35 votes
21 answers
8k views

Amount of permutations on an NxNxN Rubik's Cube

Introduction: A 3x3x3 Rubik's Cube has \$43,252,003,274,489,856,000\$ possible permutations, which is approximately 43 quintillion. You may have heard about this number before, but how is it actually ...
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33 votes
23 answers
4k views

Brute-force the switchboard

The other day, our team went to an escape room. One of the puzzles involved a board of six mechanical switches where you had to find the correct combination of on and off in order to unlock a box, ...
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32 votes
6 answers
2k views

Tic-tac-toe with only crosses

Introduction Everyone knows the game tic-tac-toe, but in this challenge, we are going to introduce a little twist. We are only going to use crosses. The first person who places three crosses in a row ...
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  • 43.4k
32 votes
7 answers
3k views

Longest domino chain

Challenge description Dominoes is a game played with tiles with two values on it - one on the left, one on the right, for example [2|4] or ...
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  • 5,442
31 votes
17 answers
3k views

Motzkin Numbers

The nth Motzkin Number is the number of paths from (0, 0) to (n, 0) where each step is of the form (1, -1), (1, 0) or (1, 1), and the path never goes below y = 0. Here's an illustration of these paths ...
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  • 41.4k
31 votes
14 answers
2k views

Iterate your way to a fraction

I recently learned from a comment by MathOverflow user pregunton that it is possible to enumerate all rational numbers using iterated maps of the form \$f(x) = x+1\$ or \$\displaystyle g(x) = -\frac ...
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  • 8,117
31 votes
15 answers
3k views

List all multiplicative partitions of n

Given a positive number n, output all distinct multiplicative partitions of n in any convenient format. A multiplicative partition of n is a set of integers, all greater than one, such that their ...
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  • 38.8k
30 votes
13 answers
4k views

Absolute Sums of Sidi Polynomial Coefficients

Background The Sidi polynomial of degree \$n\$ – or the \$(n + 1)\$th Sidi polynomial – is defined as follows. $$S_n(x) = \sum^n_{k=0}s_{n;k}x^n \text{ where } s_{n;k} = (-1)^k\binom n k (k+1)^n$$ The ...
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  • 207k
30 votes
7 answers
4k views

Which Friends characters were really the best friends?

The six main cast members of the American sitcom Friends all agreed that they would be paid the same salary throughout the run of the series (after season 2, at least). But that doesn't mean that they ...
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29 votes
31 answers
3k views

Deranged !Combinatorics: Compute the Subfactorial

The subfactorial or rencontres numbers (A000166) are a sequence of numbers similar to the factorial numbers which show up in the combinatorics of permutations. In particular the nth subfactorial !n ...
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29 votes
18 answers
7k views

Don't repeat yourself in Rock-Paper-Scissors

Upon the rumor that Codegolf will have a Rock-Paper-Scissors tournament you look into the topic of square-free words. A word made of the letters R, ...
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  • 1,717
29 votes
20 answers
4k views

Vampire Compatibility

A little known fact about vampires is that they must drink the blood of victim that has a compatible donor blood type. The compatibility matrix for vampires is the same as the regular red blood cell ...
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29 votes
4 answers
933 views

Arranging Bubbles

Note, challenge copied from question asked at math.stackexchange. Recently, I attained quite some skill at blowing bubbles. At first I would blow bubbles like this: But then things started getting ...
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  • 11.4k
29 votes
4 answers
1k views

Smallest region of the plane that contains all free n-ominoes

On Math Stack Exchange, I asked a question about the smallest region that can contain all free n-ominos. I'd like to add this sequence to the On-Line Encyclopedia of Integer Sequences once I have ...
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  • 8,117
28 votes
25 answers
2k views

Converge to a number

Your challenge is to, given a positive integer n, count up to each digit of it, giving the effect of converging on it. Basically, count up to the first digit of n by its place value (\$⌊\log_{10}\left(...
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  • 31.6k
28 votes
3 answers
1k views

Stuffing primes in a box

Your task is write a program or function that can fill a given rectangle with prime numbers. The width and height of the ...
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  • 6,732
28 votes
1 answer
2k views

Figure Out the Android Lock Pattern

Lets say you saw your friend enter his or her password into their Android phone. You don't remember how they made the pattern but you remember what the pattern looks like. Being the concerned friend ...
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  • 89.4k
27 votes
45 answers
4k views

Shortest power set implementation

Problem definition Print out the powerset of a given set. For example: [1, 2, 3] => [[], [1], [2], [3], [1, 2], [1, 3], [2, 3], [1, 2, 3]] Each element is to ...
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27 votes
21 answers
1k views

Number of Straight-Chain Alk*nes of given length

A straight-chain alk*ne is defined as a sequence of carbon atoms connected by single (alkane), double (alkene), or triple bonds (alkyne), (implicit hydrogens are used.) Carbon atoms can only form 4 ...
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  • 5,992
27 votes
17 answers
3k views

Compute the multinomial coefficient

Time for another easy challenge in which all can participate! The multinomial theorem states: The expression in parentheses is the multinomial coefficient, defined as: Allowing the terms ki to ...
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  • 4,087
27 votes
17 answers
1k views

Sums of prime factors

2013 has the prime factorization 3*11*61. 2014 has the prime factorization 2*19*53. An interesting property regarding these ...
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  • 7,219
27 votes
10 answers
1k views

Hook length product

A Young diagram is an arrangement of boxes in left-justified rows and top-justified columns. For each box, all the spaces above it and to its left are occupied. ...
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  • 138k
27 votes
4 answers
2k views

Never trust a mastermind

You probably know the game mastermind: The player tries to guess a code of 4 slots, with 8 possible colors - no duplicates this time. Let's call those colors A through H, so possible solutions could ...
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27 votes
2 answers
1k views

Golf me some cash from the ATM

The task is simple. Get me some 1000, 500 and 100 notes. How ? you might ask. Don't worry, ...
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  • 26.3k
27 votes
1 answer
1k views

Find Diffy Games

A fun game to play if you are bored is the Diffy Game. It is a one player game that is pretty simple and can consume a good deal of your time. The Diffy game works like as follows: You start with a ...
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  • 89.4k
26 votes
10 answers
3k views

The square root of the square root of the square root of the…

This code-golf challenge will give you an integer n, and ask you to count the number of positive integer sequences \$S = (a_1, a_2, \dots, a_t)\$ such that \$a_1 + ...
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  • 8,117
26 votes
23 answers
2k views

Case Permutation

Who needs to compare things case insensitively when you're able to generate every permutation of uppercase and lowercase? No one! That's the answer. No one does. Your task is to achieve this feat; ...
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  • 3,145
26 votes
24 answers
3k views

Arbitrary Randomness

Randomness is fun. Challenges with no point are fun. Write a function that, given integer input n, will output a set (unordered, unique) of exactly ...
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  • 11.2k
26 votes
6 answers
1k views

Enumerate rhyme schemes

A "rhyme scheme" is a string of letters a to z, such that the first occurrences of the characters are in ascending order (...
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26 votes
2 answers
2k views

Extend the most recent "nice" OEIS sequence: stepping stone puzzle on a grid

Today Neil Sloane of the OEIS sent out an email asking for a confirmation of the current terms, and computation of some larger terms of the latest OEIS sequence A337663 with the keyword "nice&...
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  • 8,117
26 votes
6 answers
2k views

The Coin Problem

Background The official currency of the imaginary nation of Golfenistan is the foo, and there are only three kinds of coins in circulation: 3 foos, 7 foos and 8 foos. One can see that it's not ...
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  • 42.2k
26 votes
8 answers
2k views

Generate the shortest De Bruijn

A De Bruijn sequence is interesting: It is the shortest, cyclic sequence that contains all possible sequences of a given alphabet of a given length. For example, if we were considering the alphabet A,...
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25 votes
19 answers
1k views

Just Enough Ones

Challenge Given a positive integer \$n\$, count the number of \$n\times n\$ binary matrices (i.e. whose entries are \$0\$ or \$1\$) with exactly two \$1\$'s in each rows and two \$1\$'s in each column....
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  • 6,256
25 votes
14 answers
3k views

Reuse your code!

In this challenge we try to solve two important problems at once. They are: Given integers \$a\$ and \$b\$, tell if \$a^b-1\$ is a prime number. Given integers \$a\$ and \$b\$, return \$a\choose b\$. ...
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  • 5,067
25 votes
9 answers
905 views

Generate all groupings

Let's define a grouping as a flat list, which is either: just 0 2 groupings followed by the literal integer 2 3 groupings ...
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  • 22.2k
25 votes
1 answer
2k views

Android Lock Screen

Intro You are sitting in a board room at the end of a long table. You look around and see Tim Cook, the Apple Board of Directors, the ghost of Steve Jobs, and Jack Donaghy. Apple has called this ...
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  • 351
25 votes
1 answer
823 views

Maximal discrepancy-2 sequence with minimal entropy

Background As noted in the PPCG challenge Compress a maximal discrepancy-2 sequence – which inspired this challenge – the authors of the paper Computer-Aided Proof of Erdős Discrepancy Properties ...
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  • 207k
24 votes
62 answers
3k views

Implement the hyperfactorial

The objective Given the non-negative integer \$n\$, output the value of the hyperfactorial \$H(n)\$. You don't have to worry about outputs exceeding your language's integer limit. Background The ...
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  • 349
24 votes
34 answers
2k views

Sign-Swapping Sums

Given a nonempty list of positive integers \$(x, y, z, \dots)\$, your job is to determine the number of unique values of \$\pm x \pm y \pm z \pm \dots\$ For example, consider the list \$(1, 2, 2)\$. ...
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24 votes
16 answers
3k views

Triangular domino tiling of an almost regular hexagon

Background An almost regular hexagon is a hexagon where all of its internal angles are 120 degrees, and pairs of the opposite sides are parallel and have equal lengths (i.e. a zonogon). The ...
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