# Questions tagged [combinatorics]

Combinatorics is a branch of mathematics concerning the study of finite or countable discrete structures.

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### Counting friends of friends of friends

Can you comment, review or suggest things for this? Maybe it should be broken into two methods? friends is a list of strings of ...
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### Project Euler #62 + Java 8 streams

(Inspired by this question) The digits of the cube, 41063625 (3453), can be permuted to produce two other cubes: 56623104 (3843) and 66430125 (4053). In fact, 41063625 is the smallest ...
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### Minimising the triangle

A triangle needs a good foundation. Every row in the triangle is derived from the sum of the two values below it. However, there can be no repeated values, if a value shows up more than once the ...
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### Finding the smallest cube for which exactly five permutations of its digits are cube

(Project Euler #62) The digits of the cube, 41063625 (3453), can be permuted to produce two other cubes: 56623104 (3843) and 66430125 (4053). In fact, 41063625 is the smallest cube which ...
3k views

### All upper and lowercase permutations of a string

A couple of questions: Is the algorithm wasting CPU time or memory unnecessarily? If there is something in the code that is not idiomatic Python, how to improve on that? ...
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### Permutations in C#

I have a program that will get all possible permutations of a certain equation: A + 13 * B / C + D + 12 * E - F - 11 + G * H / I - 10 == 66 Where A, B....I is a value 1-9, and where each digit is ...
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### Letter combinations of phone dial pad number

In preparation for interviews, I have written a solution to this question: Given a sequence of numbers (34128) and an input map such as a dial pad on a phone (2->[a,b,c], 3->[d,e,f], 4->[g,h,i])...
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### Safe cracker string with all combinations

Imagine a safe with a 4-digit code, and accepting a continuous stream of code entries, such that when the 4 digits are seen in the right sequence, the safe opens. Generate a short string that contains ...
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### Solver for Jumble puzzle

Here is my first attempt to solve Jumble puzzle: ...
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Original Problem Short Summary: Given a $M$ possible characters all of which must be used at least once, find the number of possible combinations of length (exactly) $N$. The first ...
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### Permutation of n lists

I have code which gives permutations for 10 lists. The code works fine for small number of lists with small number of values. Result : It should return all possible permutations. I have to store the ...
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### Solving the 'Vietnamese school maths problem'

Having read Can you do the maths puzzle for Vietnamese eight-year-olds that has stumped parents and teachers?, which presented the problem of filling in numbers 1 to 9 to satisfy  a + 13 \frac{b}{c}...
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### Unique combinations of sets with N min selections of overall set

I'm in the process of attempting to write a parser compiler. In this, sets play a major role. I'm in the 'lexical ambiguity' isolation phase, and to that, I need to yield a set of every possible ...
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### Covering array scraper and sorter

A covering array is an $N \times k$ array in which each element is a from a set of $v$ symbols, and for every $t$ columns, every possible set of $v^t$ choices of the symbols appears at least ...
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### Generating alphanumeric combinations

I was happy with my shell script that needed to generate alphanumeric combinations of length N (in this case, 3) for i in {a..z}{a..z}{a..z}; do ... Now I became ...
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### Generating combinations of n elements in groups of k

I've written this program that writes all the combinations (without repetition) of n elements in groups of k. I think the code is good, but I like to know if you have some better (or faster) ...
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### Approximate (250 over 100) permutation best fitting certain criteria

Given a list of 250 words of 4 letters each, what is the fastest way, in Python, to find a subsample of 100 words (drawn without replacement) so that the distribution of letters across the whole ...
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### Necklace counting problem-with consecutive prime constraint

This is codeEval challenge to count the number of chains possible to make with beads holding numbers. Constraint is sum of the consecutive beads of chain should be a prime number. The standard ...
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### Permutation and combination calculator

I wanted to make something for the Community Challenge, so I made a simple permutation and combination calculator. I'd like input on: The overall design - any suboptimal choices that could be ...
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### Printing all the permutations of a string in alphabetical order-ver1.1

I adapted most of the suggestions from answers for my previous question Printing all the permutations of a string in alphabetical order to rewrote my solution again. Please let me know any more ...
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### Genetic algorithm for solving a puzzle (v. 2)

Thanks to your wonderful comments regarding the first version of my code, I have been able to improve it. This new version is more readable (I guess). The problem is that it seems to me that there is ...
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### Genetic algorithm for solving a puzzle

Edit. Version 2. I am working on a genetic algorithm in order to solve a little "puzzle." Given a text file with N rows with 4 int each, the idea is to ...
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### Printing all the permutations of a string in alphabetical order

It is a code eval challenge question https://www.codeeval.com/open_challenges/14/. My answer was accepted, and I modified my code again to improve it. Please let me know is it possible to improve it ...
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### Calculate number of possible combinations to make \$100 with all US denominations

I know that this could be made much more efficient, I am just not sure as to how. This program ignores the 100 dollar denomination to cut computation time in half and just starts ...
162 views

### Generating letter combinations

It's about two months since I posted this question on CodeReview. I've been working hard to improve the code posted there and came up with the following. ...
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### Consecutive Primes challenge at CodeEval.com and memory allocation issue

I finally solved this challenge with using recursion and figured out why the automatic scorer wasn't passing my previous solutions. I was using NSMutableArray to ...
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### Consecutive Primes challenge at CodeEval.com

I am working through a new challenge on CodeEval.com and I think I am getting a correct result but I can't get through their grader. I am not exactly sure where error is. I didn't post this on stack ...
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### Fast canonical indexing of elements in array

Given two arrays of elements: aaabbccdefaa zzzccddafgzz I want to check whether these two arrays are equivalent up to a relabeling. My strategy is to map them into a canonical form and then ...
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### Is_permutation() on sequence containers in O(n) complexity - follow-up

This question is a continuation of my previous question: Check is_permutation on a pair of same sized arrays. I improved the implementation to generalize for all sequence containers. Please let me ...
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### Modified Coupon Collector's Problem in Java

What is Coupon Collector's Problem? In probability theory, the coupon collector's problem describes the "collect all coupons and win" contests. It asks the following question: Suppose that there is ...
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### Approximately putting integers into buckets such that the sum of bucket differences is minimized

I have the need for load balancing in my MSD-radix sort routine and doing it optimally is not an option. So I tried to play with two simple routines for putting integers into a prespecified amount of ...
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Question: ...
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### Two permutation iterators for classes in Python with and without a generator

I am just testing out a couple of small classes for learning purposes. Here is my current working code: ...
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### Finding Lottery odds - Topcoder #268

I have implemented the question #268 from Topcoder. The question is that there are 4 conditions for a lottery ticket: CHOICES - numbers available (10 ≤ CHOICES ≤ 100) BLANKS - number of numbers that ...
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### String manipulations (reverse a string by characters, permutation)

I'm trying to solve simple Java problems in various ways. Could anyone comment about the code, like how it could be better, and what could be a problem? ...