## Thursday, August 30, 2012

### UVA 10227 - Forests

http://uva.onlinejudge.org/external/102/10227.html

If a tree falls in the forest, and there's nobody there to hear, does it make a sound?

A forest contains $N$ trees and $P$ persons. Each person has an opinion about a set of trees they heard fall. Your task is print the number of different opinions among the $P$ persons.

Let's first try to get some intuition about the situation that we are dealing with:

As we can see in the previous image person #1 and #3 has the same opinion, they heard tress $\{2, 3\}$ fall. In the case of person #2 he heard the trees $\{2, 4\}$ falls, this give us a total of 2 different opinions, $\{2,3\}$ and $\{2,4\}$.

The key observation to solve this problem is to notice that the persons are divided into connected components according to the set of trees they heard fall. For the previous example we have two connected components:

Once again we are going to need the Disjoint Sets data structure to solve this problem. Initially each person has his own opinion, for each pair of people we check if they shared the same opinion with somebody else in the group, if they do we used the operation $union\_set(x,y)$ to merged this two persons into a connected component, otherwise, continue with the next person. Our answer is the number of connected components after all the $union\_set(x,y)$ operations.

Finally, something important to remember is that for this problem "having no opinion also counts as having an opinion". Which means that persons that do not heard any trees fall, they should be group together also in one group.

## Tuesday, August 28, 2012

### UVA 10178 - Count the Faces

http://uva.onlinejudge.org/external/101/10178.html

You are given a planar graph. A planar graph is one that can be drawn on a plane in such a way that there are no “edge crossings,” i.e. edges intersects only at their common vertices. Your task is to print the number of faces in that graph.

For example the following graph has 6 faces:

To solve this problem the first thing that we need to do is understand about the faces and how these are created. To simplify things let's forget about the outer face (face #6) and focus in the others. It is not hard to see that the rest of the faces are enclosed regions of 3 or more points, in other words there are cycles. This implies that the problem of counting the cycles in an undirected planar graph is analogous to the number of faces in the graph, with the only exception that we should not use an edge more than one time.

To count the cycles in an undirected planar graph we are going to use the Disjoint Sets data structure.

In order to better illustrate this ideas let's take a look to an example:

Imagined that we are using the operation $union\_set(x,y)$ for all the edges in the previous graph. Let's assume that we connect the edges in the following order $(A, B)$ - $(B, D)$ - $(D, C)$ - $(C, A)$ - $(C, E)$ - $(E, G)$ - $(G, A)$. A green edge represent a connection between two nodes were each of the nodes belong to a different connected component. In those cases we do not have a cycle. A red edge or cycle appear just in the cases were we connect two nodes that already belongs to the same component. In those cases we should increment the faces counter. Note that this idea also works where you have more than one connected component in the graph.

Once we are done with all the edges our answer is the number of cycles founded in the graph plus one. Discussing this problem with my friend cjoa2 he told me that the mathematician Leonhard Euler discover a formula to calculate the number of faces in a planar graph.
Were v and e represent the total count of vertices and edges in the graph. For example given the previous graph we have the following:
There is just a little issue in the case that we have different connected components if we applied the formula for each component we are going to end up over counting the number of outer faces. Let's see an example that illustrate this problem:

If we calculate formula for each component and add up the results we get that the total number of faces in the those planar graphs is 4, which is clearly incorrect... the correct answer should be 3:

To be able to solve this issue we are going to calculate the number of faces in each component with the following formula:
Once we sum the result of each component, we add one to our final answer, corresponding to the outer face of all the connected components.

Finally, To implement this idea we use a depth first search method that give us the total amount of vertices and edges in each connected component and used the previous explained formula.

## Wednesday, August 15, 2012

### A. Tiling with Hexagons

http://www.codeforces.com/contest/216/problem/A

You are given three positive integers $a$, $b$ and $c$ $(2 \leq a, b, c \leq 1000)$. The integers $a$, $b$, $c$, $a$, $b$ and $c$ form a six side hexagonal shape. The hexagonal shape is tiled with hexagonal tiles. Your task is to print the total number of tiles in the hexagonal shape.

Let's first consider the case where $a = 2$, $b = 3$ and $c = 4$. Because this kind of "weird" hexagonal figures are counter-intuitive we are going to transform the previous figure into better known one, a rectangle:

Given this example is pretty easy to guess an answer, we just multiply the base x height and subtract the total amount of red hexagonal tiles and we are done. But hold on a second, there are two things we are missing... first, what is the length of the base and the height, and also how to calculate the total amount of red hexagonal tiles.

Let's analyze two more examples to see if we can spot the pattern, now consider the case when $a = 3$, $b = 3$, $c = 4$ and $a = 4$, $b = 3$, $c = 4$:

Now we are getting somewhere... if you look carefully the base of the new rectangle is equal to the expression $c + a - 1$ and the height is equal to the expression $b + a - 1$. The red tiles as we can see are triangular numbers, to be precise the $T_{a-1}$ where $T_{i}$ denotes the $i$-th triangular number, and because we have two of those we subtract $2 * T_{a-1}$ to get the answer. So let's denote $h(a,b,c)$ as the total amount of hexagonal tiles, we got the following expression:

### B. Forming Teams

http://www.codeforces.com/contest/216/problem/B

You are given $N$ people and their relationship, you want to form two teams with equal number of members. Unfortunately some people are archenemies, because you don't want bad chemistry in any of the teams you decided not to form a team which has  archenemies on it. To be able to accomplish this task you may have to left out some people. Your task is to print the minimum amount of people that you have to left out to form a team in the described manner.

Well something that as a mathematician, programmer or whatever you consider yourself should always remember is "you do not need to do exactly what the problem is telling you to solve it", This may sound pretty obvious but sometimes is pretty hard to know when to follow this principle. In this case, we do not need to form the teams to know how many people are left out. Instead we are going to tried to came with the solution by spotting the archenemies relationship in a graph.

Let's imagine that the color blue represent the first team and red the second one.  The following graph shows a possible team distribution of four people:

Because person #1 and person #2 are archenemies the best that we can do is to assigned them to different teams, in a similar manner the same applies for the pairs $(2, 3)$ and $(3, 4)$.

Problems arise when we attempt to join members that form a chain (cycle):

No matter how we assigned the people in the two teams we need to left out one person (in this case the person #3 marked in grey) to make thinks workout. At this point we may feel tempted to conjecture that the same applied for all the cycles. Unfortunately that is wrong, an easy counter-example is the following case:

Looking carefully to the previous graphs is easy to spot the pattern, every time we find an odd cycle at least one person should be left out. In case that you still doubt the previous statements let's also include a five people cycle example:

Let's recall that the problem statement establish that one person can not have more than two archenemies. So we are going to have graphs like the previous discussed.

To get our answer we should count the total number of odd cycles, let's call it $C$, if $N - C$ is odd we should left out another person, this is because the number of persons in each team should be strictly equal, in other words divisible by two.

Finally, to implement the solution I used the Disjoint Set data structure. The idea is the following, We first iterate over all the edges of archenemies, for each of them we tried to connected them, if we fail means that there were previously merged, which means we found a cycle, and we should increment the number of left out persons, otherwise, add another node to some connected component and continue the same process.