# Questions tagged [algorithm]

A puzzle related to algorithmics (a main concept in computer science).

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### Three knights searching for a princess in a castle

There is a castle with rooms arranged in a 4x4 square grid. The princess living in the castle sleeps in a different room each night, but always one adjacent to the one in which she slept on the ...
310 views

### Two knights searching for a princess in a castle

There is a castle with rooms arranged in a 3x3 square grid. The princess living in the castle sleeps in a different room each night, but always one adjacent to the one in which she slept on the ...
66 views

### Arranging people to be in a group with every other person at least once

There are 12 people. These people start in 4 groups - each with 3 people. They swap groups 4 times, so they are in a total of 5 groups. Is it possible for each person to be in a group with every other ...
164 views

### Hunting for treasure in 1-d grid [duplicate]

In a castle, there are N rooms in a line. Each room has a door to the two neighbouring rooms (or the one neighbouring room, for the rooms at either end). A treasure is hidden in one of the rooms. ...
177 views

### How to find number of times sequence element $1$ is approached and from where?

Consider a sequence $1,-1,-1,-1,-1,-1,...,-1$. Start at the first element and move down the sequence according to the following rules: 1) If you jump from a $-1$ to another $-1$, turn the latter into ...
249 views

### The CompuTaria Quest - A young Wizard's journey

Once Upon A Time In A Land FAR, FAR Away... Our adventure begins in the enchanted land called "CompuTaria". You are, "Binaryian" a 3rd year Wizard In Training. Today began like any other day - you ...
318 views

### Cube Scrambling

Take a normal 3x3 rubiks cube (the Classic unmodded cube). I know the WCA method for scrambling is to pick a random (valid) cube state and calculate a sequence of moves to go from solved to that state,...
255 views

### Minimize sum of guesses to win lottery

Ten boxes are given with $a_1,a_2,a_3,a_4,a_5 ......a_{10}$ number of balls in them respectively .These boxes are randomly ordered but $a_1,a_2 .....a_{10}$ is told.We can arbitrary select a box and ...
348 views

### Avoiding Incest and Intergenerational Mating

On the planet of Caturn, God created $m$ male cats and $f$ female cats, each with zero stripes. At any time, two cats of opposite genders can mate, giving birth to a cat with a gender of the parents' ...
2k views

### Finding Round Integer

An integer is round if it is greater than $0$ and the sum of its digits in decimal representation is a multiple of $10$. Find an optimal procedure to compute the $N^\text{th}$ smallest round integer. ...
1k views

### Finding numbers having exactly two distinct digits

We have $10^K$ road signs (numbered 0 through $10^K−1$). For each valid $i$, the sign with number $i$ has the integer $i$ written on one side and $10^K−i−1$ written on the other side. We need to ...
18k views

### Why does this solution guarantee that the prince knocks on the right door to find the princess?

I found this puzzle online: On the top floor of a castle lives a princess. The floor has 17 bedrooms arranged in a row. Each bedroom has doors connecting to the adjoining bedrooms as well as to the ...
292 views

### Savage Road Signs (Part 2)

Please read part 1 or this might be confusing Since part 1, you have replaced the stolen stickers and your daughter has forgiven you. The highway ended up being a full 700km long, so you are happy ...
2k views

### Wolves and sheep

All the sheep were living peacefully in the Land of Shewo. But suddenly they were struck by a danger. A few wolves dressed up as sheep entered the territory of Shewo and started killing the sheep one ...
231 views

### $4\times4\times4$ Rubiks cube Top Layer Issue

The only way i am able to solve is to jumble the whole cube again. If you know the way to solve pls give images or algorithm.
203 views

### SafeCracker #2 - The Mission Continues

Thanks to an alert StackE user, we were able to get the first safe open. Mission Details This next safe is in a former employee's house. He is gone for now, so we have no time to spare. We weren't ...
926 views

### SafeCracker #3 - We've Been Blocked [closed]

Our Last Safe To Conquer Ok, team, this one might give us a little trouble. Nothing we can't handle though... let's go... This safe model is "The NevaCrak 9000" - the most secure safe there is. ...
145 views

### SafeCracker #1 - Look who's talking

The Plot You work for a tech company in Tampa, FL. Recently there was a break-in, and the company's trade secrets were stolen. Company PI have tracked the briefcase containing the secrets to the ...
90 views

### The NANNP Grid ( Not another Next Number Puzzle )

Simply figure out the last row...
854 views

### Blindfold Bingo

Congratulations, you have been selected from among the audience to compete in the fabulous Blindfold Bingo challenge! (cue annoying, yet catchy theme music.) Here you see our perfectly ordinary Bingo ...
2k views

### 23 Clones and Two Lightbulbs

Rise and shine, Mister Bond. James awoke to find himself yet again imprisoned by an evil genius. At least, that's what he presumed. He was alone in a room with a strange device: it consisted of a ...
186 views

### Find the result of this dual key cipher

I came up with a neat little cipher that I'm calling the dual key cipher. The basic premise is that there are two cipher keys that, when used together can decode the cipher text against a given ...
149 views

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### Shortest path for delivering packages [duplicate]

A version of the Travelling Salesman Problem, but with certain points. We are talking about a town where 8 canals run from the middle boathouse into the city. You can only travel one way on the green ...
288 views

### Puzzles like Sokoban?

I am looking for some puzzles like Sokoban or 15-puzzle but more difficult to solve and satisfy the following requirements: The number of possible moves at each step should be limited, let's say < ...
1k views

### Let's deal with “Names in Boxes” Safely

I met a puzzle, which is basically well known 100 Prisoners' Names in Boxes , but... it has an algorithm, which gives prisoners 100% chance to survive! This is achieved by a small change in the ...
354 views

### 20-Questions on a grid

Posted this on /r/puzzles too, none have given the right strategy as of now :) There is a square grid of width and height $m$. Three facts about the grid: Each cell has a value, and no two cells ...
821 views

### What is the optimal first move in Mastermind?

For a given number of positions/holes and a given number of colors, which initial guess eliminates the most possibilities for the solution at a minimum? For 4 positions, it's AABB. What is it for 5, ...
164 views

### It's getting harder and harder for my friends to like me

What is the next number in this sequence? 2, 3, 6, 7, 9, 18, 25, 27, 54, 73, 97, 129, 171, 231, 313, 327, 649, 703, ... ? Hint 1 Hint 2 This is my first puzzle and I'm new to PSE, so feedback is ...
428 views

### Ten users, three lights but it's a chatroom

This is a variant of the classic problem of m prisoners n lights. (Example may be found here but note that the requirements are not exactly the same) The goal for you, however, is different. You want ...
122 views

### Ten users, ten deadly collars and three lights [duplicate]

One day, 10 puzzling.SE users, including you, find yourselves in a horrible prison. Each of you has a deadly collar, presently lethal. It is safe to take off only when all of the 10 collars have been ...
180 views

### A couples therapy mixer situation

During couples-group-therapy, where two therapist and multiple couples consisting of two people each are present and we all sit on chairs forming a circle in the middle of the room. We often discuss ...
36k views

### What is the Official notation on rubik's revenge(4x4)?

When solving a 4x4 cube you have specific notation to inner and outer lines, like R = Clockwise outer right and r = Clockwise ...
1k views

### The Generous Taco stand

There are a group of people standing in line for a taco stand. This taco stand is special in that it's slogan is "The older you are, the more you get!". To make sure though that there isn't any funny ...
90 views

### How can these people cross the bridge? [duplicate]

There are 4 people that must cross an old dilapidated bridge at night, All four walk at different speeds: There is a young teenager that can cross the bridge in 1 minute. There is teenager's older ...
5k views

You are a spy trying to break into an enemy facility. The back door is protected by an electronic keypad lock. You know that this particular lock is opened by a four digit code. Any stream of button ...
269 views

### Scheduling Meetings

I came across this problem in real life and thought it could be made into an interesting puzzle. I will enjoy seeing how my eventual solution could be improved! Here's the situation. There ...
1k views

### Algorithm for solving the permutation puzzle Tricky Animals

I have developed a game called Tricky Animals where the goal is to reorder the animals in the right order with three permutations: $A = (1,2), X = (2,3,..,n-1), B = (n-1, n)$. The game can be played ...
217 views

### When will inverting an algorithm allow it to function the same as the original?

Let's say I have this zperm algorithm: R' U' R U' R U R U' R' U R U R2 U' R' U2 Now, let's suppose I invert it. U2 R U R2 U' R' U' R U R' U' R' U R' U R Assuming the cube is in a state of its pll ...
781 views

### Is my 5x5 cube on an “illegal” position?

I've never met this postion before, and i'm starting to think it's an "illegal" one because i lend it to a friend before and he said he had to glue some fallen stickers. Am i missing an alorithm or do ...
219 views

### Finding all possible solutions of a japanese sums puzzle

I'm trying to think of an algorithm that gives out all possible solutions to a japanese sums puzzle. It's not an a college assignment or anything, I'm giving puzzles to my cousin for fun, but want to ...
312 views

### Is there a name for this Rubik's cube algorithm?

$RUR'U RU2R'$ I use this algorithm to: permute UL and UF edges (messing up corners) orient LL from a fish (possibly using its mirror) orient pairs of LL corners by combining above algorithm with its ...
230 views

### Figure out when a data usage cycle restarts

Your cell phone carrier sends you a text message when you have reached your data limit (which you know) for the month, after which you are charged for extra data usage. Question 1: You want to ...