So far, we talked about networks in general. Now, I want to talk about a very specific kind of network that is also very important. In this kind of network is called Trees or also Hierarchies. So, let me first describe what a tree is and then I'm going to talk about how to represent it visually. So, the basic idea of a tree is that it's a specific kind of network that is organized as a hierarchy. So, there is one single node that is the starting point and it's called the root node, and following the root node, there can be two or more child or children and the structure is organized in a way that every node can have between zero and multiple children and every child can have only one parent, similar to what you see in this picture in front of you. As I said, this kind of data structure is called the Tree and it can represent hierarchies and that's the reason why it's so important and pervasive. It turns out that there are lots of real-world phenomena and objects that can be described using a tree. Let me give you a few examples. For instance, a file system within your computer as you may already know it's a tree, right? So, you have one root folder and then within this root folder there are many other folders and within each of these folders there is another folder or files and so on. That's a typical example of a tree hierarchy. It's also very much used in biology as we will see later. There is a thing called phylogenetic trees, which is a way to describe evolution of organisms. That's also an example of a tree, starting from one organisms there are several branches and every times organisms branch out they split into new organisms. Another interesting observation is that anything related to geography or time is also hierarchical. Think about the structure of time, you can go say from seconds to minutes, hours, days, weeks, months, and so on and senior structure exist in geography, right? So, you can go from let's say country to regions within a country's, cities within a country's, zip codes within cities and so on. What else? Organizational structures within a company are trees or hierarchies as well as language. Language structure as we will see later on in an example. Every language can be described as a tree. When we talk about how to visualize trees there are two main categories or solutions. The first one, once again is a specific kind of node link diagram. The same way we saw node link diagrams as a general purpose solution for networks, we have the same solution here. We can represent trees as a collection of nodes represented by visual marks that are connected by lines that represent the relationship between these two nodes, and then there is another broad category of solutions that I call containment or partitioning and that's going to be later on when I introduced this class is going to be much clearer, what I'm talking about. For now, let's focus in the next video on all those solutions that are based on node link diagrams.
