Tag Archives: puzzle

Clique Problem Puzzles

I still remember how I felt when I was first introduced to NP-Complete problems. Unlike the material I had learned up to that point, there seemed to be such mystery and intrigue and opportunity surrounding these problems. To use the example from Garey and Johnson’s book “Computers and Intractability: A Guide to the Theory of NP Completeness”, these were problems that not just one researcher found difficult, but that a number of researchers had been unable to find efficient algorithms to solve them. So what they did was show that the problems all had a special relationship with one another, and thus through this relationship if someone were to discover an algorithm to efficiently solve any one of these problems they would be able to efficiently solve all the problems in this class. This immediately got my mind working into a world where I, as a college student, would discover such an algorithm and be mentioned with the heavyweights of computer science like Lovelace, Babbage, Church, Turing, Cook, Karp and Dean.

Unfortunately I was a student so I did not have as much time to devote to this task as I would have liked. In my spare time though I would try to look at problems and see what kind of structure I found. One of my favorite problems was, The Clique Problem. This is a problem where we are given an undirected graph and seek to find a maximum subset of nodes in this graph that all have edges between them, i.e. a clique (Actually the NP-Complete version of this problem takes as input an undirected graph G and an integer k and asks if there is a clique in G of size k).

Although I now am more of the mindset that there do not exist efficient algorithms to solve NP-Complete problems, I thought it would be a nice project to see if I could re-create this feeling – both in myself and others. So I decided to write a program that generates a random undirected graph and asks users to try to find a maximum clique. To test users answers, I coded up an algorithm that works pretty well on smaller graphs, the Bron-Kerbosch Algorithm. This algorithm uses backtracking to find all maximal cliques, which then allows us to sort them by size and determine the largest.

Users should click on the numbers in the table below the canvas indicating the nodes they wish to select in their clique (purple indicates that the node is selected, gray indicates that it is not). Once they have a potential solution, they can press the “Check” button to see if their solution is optimal. If a user is having trouble and simply wishes to see the maximum clique, they can press the “Solve” button. And to generate a new problem, users can press the “New Problem” button.

So I hope users have fun with the clique problem puzzles, and who knows maybe someone will discover an algorithm that efficiently solves this problem and become world famous.

Unidirectional TSP Puzzles

As we’ve entered the late spring into early summer season, I’ve found myself wanting to go out more to sit and enjoy the weather. One of these days recently I sat in the park with a good book. On this occurrence, I decided not to go with a novel as I had just finished “Incarceron“, “The Archer’s Tale“, and “14 Stones” – all of which were good reads, but I felt like taking a break from the novels.

Just as a side note, 14 Stones is a free book available on smashwords.com and I’ve now read about 6 books from smashwords.com and haven’t been disappointed yet. My favorite is still probably “The Hero’s Chamber” because of the imagery of the book, but there are some well written ebooks available there by some good up and coming writers for a reasonable price, with some being free.

 

So with the desire to read, but not being in the mood for novels I decided to pick up one of my non-text but still educational books that make me think. This day it was “Programming Challenges“. I browsed through the book until I found one that I could lay back, look at the water, and think about how to solve it.

 

The programming puzzle the peaked my interest was called “Unidirectional TSP”. We are given a grid with m rows and n columns, with each cell showing the cost of using that cell. The user is allowed to begin in any cell in the first column and is asked to reach any cell in the last column using some minimum cost path. There is an additional constraint that once a cell is selected in a column, a cell in the next column can only be chosen from the row directly above, the same row, or the row directly below. There is a javascript version of this puzzle available here.

Fundamentally, the problem is asking for a path of shortest length. Many shortest length problems have a greedy structure, but this one gained my interest because the greedy solution is not always optimal in this case. So I took a moment to figure out the strategy behind these problems. Once I had that solution, I decided that it would be a good program to write up as a puzzle.

 

In this puzzle version, users will click the cells they wish to travel in each column in which case they will turn green (clicking again will turn them white again). Once the user clicks on a cell in the last column, they will be notified of whether or not they have chosen the minimum path. Or if users are unable to solve a puzzle, the “Solution” button can be pressed to show the optimal path and its cost. 

Triangle Sum Puzzle

This is probably a consequence of being a mathematician, but I have always enjoyed number puzzles. I think that there is a general simplicity and universality in numbers that are not present in things like word puzzles, where the ability to reach a solution can be limited to the vocabulary of the user.

The fact that these are puzzles and not simply homework exercises also helps because we often find people sharing difficulties and successes stories over the water cooler or at the lunch table. The fact that many of these math puzzles can teach some of the same concepts as homework problems (in a more fun and inclusive way) is generally lost on the user as their primary interest is generally on solving the puzzle in front of them, or sometimes solving the more general form of the puzzle.

Today’s post is about a puzzle that was originally shared with me over a lunch table by a friend who thought it was an interesting problem and asked what I thought about it. I didn’t give the puzzle much further thought (he had correctly solved the puzzle) until I saw it again in “Algorithmic Puzzles” by Anany Levitin and Maria Levitin. It was then that I thought about the more general form of the puzzle, derived a solution for the problem, and decided to code it up as a script for my site.

Below is a link to the puzzle:

We have a set of random numbers arranged in a triangle and the question is to find the path of maximum sum from the root node (the top node) to the base (one of the nodes on the bottom row) with the rules that
(1) Exactly one number must be selected from each row
(2) A number can only be selected from a row if (a) it is the root node or (b) one of the two nodes above it has been selected.

For the sample
So for the sample problem in the picture, the maximal path would go through nodes 57, 99, 34, 95, and 27.

For more of these puzzles check out the script I write here and be sure to let me know what you think.

The Bridge Crossing Problem

Most puzzles are fun in their own right. Some puzzles are so fun that they have the added benefit that they are likely to come up in unexpected places, like maybe in a job interview. I was recently reading a paper by Günter Rote entitled “Crossing the Bridge at Night” where Rote analyzes such a puzzle. Upon finishing the paper, I decided to write a script so that users could see the general form of this puzzle.

The problem can be stated as follows: There is a set of people, lets make the set finite by saying that there are exactly n people, who wish to cross a bridge at night. There are a few restrictions that make crossing this bridge somewhat complicated.

  • Each person has a travel time across the bridge.
  • No more than two people can cross the bridge at one time.
  • If two people are on the bridge together, they must travel at the pace of the slower person.
  • There is only one flashlight and no party (of one or two people) can travel across the bridge without the flashlight.
  • The flashlight cannot be thrown across the bridge, and nobody can go to the store to purchase another flashlight

The image above shows the optimal solution when the 4 people have travel times of 1, 2, 5, and 10. The script I have written allows users to work with different numbers of people with random travel times. Give it a try and see if you can spot the patterns in the solution.

Magical Squares Game

Whether introduced as children in elementary school, as adults in the workplace, or somewhere in between, the concept of magic squares has fascinated people for centuries; The Wikipedia article has discoveries of magic squares dating back to 650 B.C. in China.

Magic Squares of size n (for n >= 3) are n by n grids where the numbers 1, 2, …, n2 are specially arranged such that the sum of each row, column, and diagonal all sum to the same number. Below are two examples of magic squares of size 3 and 4.

8 3 4
1 5 9
6 2 7
4 1 16 13
15 11 2 6
10 14 7 3
5 8 9 12

Notice first, that in the first square all the numbers 1, 2, 3, .., 9 are used in the square. Likewise, the numbers 1, 2, …, 15, 16 are used in the second square. Second notice that each row, column and diagonal sums to 15 in the first square and 34 in the second square.

I have recently published a puzzle that is based on the concept of magic squares. There are some slight differences though.
– First, instead of using the numbers 1, 2, …, n2 a random set of numbers are generated.
– Second, the rows and columns each have a desired sum that we would like the numbers in the row/column to sum to.

These two changes allow for a puzzle concept to be formulated based on the magic square concept. Users take turns swapping elements until the numbers in each row and column sum to the number in their goal cell, which is located in the last column or row of the grid. Above is an image of a solved puzzle.

Users can determine their progress the numbers in the next-to-last column, which tells the current sum of the numbers in that respective row or column.

To swap two numbers, first click on a cell with one of the two numbers in it. The cell should then turn red. Then click on the cell with the other number in it and the numbers should swap. If you click on the same cell twice no action should take place (except for the cell to turn red and then blank again).

Take a moment to check out the puzzles and let me know what you think.

Sudoku Program Updates

Picture of Sudoku Page

Here in DC, we recently had an unexpected snow day. By the word unexpected, I don’t mean that the snow wasn’t forecast – it was definitely forecast. It just never came. However due to the forecast I decided to avoid traffic just in case the predictions were correct. So while staying at home, I began thinking about some things that I’ve been wanting to update on the site and one thing that came up was an update to my Sudoku program. Previously, it contained about 10000 sample puzzles of varying difficulty. However, I told myself that I would return to the idea of generating my own Sudoku puzzles. I decided to tackle that task last week.

The question was how would I do this. The Sudoku solver itself works through the dancing links algorithm which uses backtracking, so this was the approach that figured as most likely to get me a profitable result in generating new puzzles (I have also seen alternative approaches discussed where people start with an initial Sudoku and swap rows and columns to generate a new puzzle). The next question was how to actually implement this method.

Here is an overview of the algorithm. I went from cell to cell (left to right, and top to bottom starting in the top left corner) attempting to place a random value in that cell. If that value can be a part of a valid Sudoku (meaning that there exists a solution with the current cells filled in as is), then we continue and fill in the next cell. Otherwise, we will try to place a different value in the current cell. This process is continued until all cells are filled in.

The next step was to create a puzzle out of a filled in Sudoku. The tricky about this step is that if too many cells are removed then we wind up generating a puzzle that has multiple solutions. If too few cells are removed though, then the puzzle will be too easy to solve. Initially, I went repeatedly removed cells from the locations that were considered the most beneficial. This generally results in a puzzle with about 35-40 values remaining. To remove additional cells, I considered each of the remaining values and questioned whether hiding the cell would result in the puzzle having multiple solutions. If this was the case, then the cell value was not removed. Otherwise it was. As a result I now have a program that generates Sudoku puzzles that generally have around 25 hints.

You should give it a try.

Nonogram Puzzles

A friend introduced me to a type of puzzles called Nonograms and I enjoyed them so much that I wrote a script that automatically generates these puzzles.

Nonograms are grid puzzles based on discovering the hidden pattern based on the clues provided. This hidden pattern is the answer to the question of which cells of this grid should be shaded black, and which ones should be shaded grey. The clues come in the form of lists at the beginning of each row and column. The list represents the sizes and order of the groups of shaded cells in that line. For example, if there is a list with the numbers “4 2″, then it says that the group has 4 shaded cells, then one or more unshaded cell, then two shaded cells. Because 4 becomes before 2 in the list, the 4 shaded cells would become before the 2 shaded cells in the line of the grid. Also there must be at least one unshaded cell in between the groups because if there wasn’t, then the “4 2″ list would actually be a group of 6 shaded cells.

So have fun with these and let me know what you think.

Other Blogs that have covered this topic:
MINIGAMESCLUB

Shade The Cells Puzzle

A friend described this puzzle to me and I enjoyed it so much that I just had to write a script so that I could play it more.

The rules of this puzzle are simple. Cells can be in one of three states:

An UNSHADED (white) cell means that you have not considered this cell yet.
A DARK GREY SHADED cell means that the sum of the dark grey shaded cells in that row and column must equal the number in that cell.
A LIGHT GREY SHADED cell means that the sum of the dark grey shaded cells in all the connected cells must equal the number in that cell.

I Hope you Enjoy

How Could You Possibly Love/Hate Math?

Growing up, I never really liked math. I saw it as one of those necessary evils of school. People always told me that if I wanted to do well and get into college, I needed to do well in math. So I took the courses required of a high school student, but I remember feeling utter confusion from being in those classes. My key problem was my inquisitive nature. I really didn’t like being “told” that certain things were true in math (I felt this way in most classes). I hated just memorizing stuff, or memorizing it incorrectly, and getting poor grades because I couldn’t regurgitate information precise enough. If this stuff was in fact “true”, I wanted to understand why. It seemed like so much was told to us without any explanation, that its hard to expect anybody to just buy into it. But that’s what teachers expected. And I was sent to the principal’s office a number of times for what they called “disturbing class”, but I’d just call it asking questions.

At the same time, I was taking a debate class. This class was quite the opposite of my math classes, or really any other class I’d ever had. We were introduced to philosophers like Immanuel Kant, John Stuart Mill, Thomas Hobbes, John Rawls, etc. The list goes on and on. We discussed theories, and spoke of how these concepts could be used to support or reject various propositions. Although these philosophies were quite complex, what I loved was the inquiries we were allowed to make into understanding the various positions. Several classmates and I would sit and point out apparent paradoxes in the theories. We’d ask about them and sometimes find that others (more famous than us) had pointed out the same paradoxes and other things that seemed like paradoxes could be resolved with a deeper understanding of the philosophy.

Hate is a strong word, but I remember feeling that mathematicians were inferior to computer programmers because “all math could be programmed”. This was based on the number of formulas I had learned through high school and I remember having a similar feeling through my early years of college. But things changed when I took a course called Set Theory. Last year, I wrote a piece that somewhat describes this change:

They Do Exist!

Let me tell you a story about when I was a kid
See, I was confused and here's what I did.
I said "irrational number, what’s that supposed to mean?
Infinite decimal, no pattern? Nah, can't be what it seems."
So I dismissed them and called the teacher wrong.
Said they can't exist, so let’s move along.
The sad thing is that nobody seemed to mind.
Or maybe they thought showing me was a waste of time.

Then one teacher said "I can prove they exist to you.
Let me tell you about my friend, the square root of two."
I figured it'd be the same ol' same ol', so I said,
"Trying to show me infinity is like making gold from lead"
So he replies, "Suppose you're right, what would that imply?"
And immediately I thought of calling all my teachers lies.
"What if it can be written in lowest terms, say p over q.
Then if we square both sides we get a fraction for two."

He did a little math and showed that p must be even.
Then he asked, "if q is even, will you start believing?"
I stood, amazed by what he was about to do.
But I responded, "but we don't know anything about q"
He says, "but we do know that p squared is a factor of 4.
And that is equal to 2 q squared, like we said before."
Then he divided by two and suddenly we knew something about q.
He had just shown that q must be even too.

Knowing now that the fraction couldn't be in lowest terms
a rational expression for this number cannot be confirmed.
So I shook his hand and called him a good man.
Because for once I yould finally understand
a concept that I had denied all my life,
a concept that had caused me such strife.
And as I walked away from the teacher's midst,
Excited, I called him an alchemist and exhaled "THEY DO EXIST!"

Aside from its lack of poetic content, I think that many mathematicians can relate to this poem, particularly the ones who go into the field for its theoretic principles. For many of us, Set Theory is somewhat of a “back to the basics” course where we learn what math is really about. The focus is no longer on how well you can memorize a formula. Instead, its more of a philosophy course on mathematics – like an introduction to the theory of mathematics, hence the name Set Theory.

The poem above focuses on a particular frustration of mine, irrational numbers. Early on, we’re asked to believe that these numbers exist, but we’re not given any answers as to why they should exist. The same could be said for a number of similar concepts though – basically, whenever a new concept is introduced, there is a reasonable question of how do we know this is true. This is not just a matter of practicality, but a necessity of mathematics. I mean I could say “lets now consider the set of all numbers for which X + 1 = X + 2″, but if this is true for any X, then it means that 1 equals 2, which we know is not true. So the set I’d be referring to is the empty set. We can still talk about it, but that’s the set I’d be talking about.

So why is this concept of answering the why’s of mathematics ignored, sometimes until a student’s college years? This gives students a false impression of what math really is, which leads to people making statements like “I hate math”, not really knowing what math is about.