So you've been hearing from Christine about linked lists. And as a data structure, it's not such a complicated data structure. It's just got these nodes that are linked together one after the other. But it turns out that implementing linked lists is really quite tricky. And so this concept challenge that we have for you will help you think through some of the details of the implementation, so you're ready to do it when you get your programming assignment. So as you know, with a concept challenge, we'll be posing a question and you'll pause and think through the answer and solve it yourself. Then you have the opportunity to discuss your solution with people around you or in the discussion forums. We'll show you some UC San Diego learners thinking through the same ideas. You'll answer the question again. Hopefully with a little bit more confidence behind your answer now. And then confirm your understanding as we come together and walk through the solution. So let's get started setting off the problem. In this problem, we're going to tweak the data structure that Christine talked about a little bit. We'll be implementing a singly linked list. In the core videos, the linked list data structure that you saw was doubly linked. And we're modifying just a little bit so we have a cleaner example for this concept challenge, and also so that we don't take all the fun out of the implementation when you go ahead and do the programming assignment. So just to point out the differences in the singly linked list, we only have a sentinel object at the head of the list. And we don't have a tail sentinel. And then when we think about what's contained inside each of the object, we have next references, we don't have previous references. All right, so we have a head object and then we point forward choosing next pointers. So this is our abstract representation of the singly linked list. And what this concept challenge really is about is going from the code to these pictures of the nodes linked together and seeing how the code relates to those objects. So the code that we've written is for defining a class SSLNode, which is a single node in a singly linked list. And in this particular implementation, we're gonna have three different constructors that can be used to create a node. And what I'd like you to think about is what happens when we use these constructors to create some nodes and try to link them together. >> Hi, my name is Paul >> I'm Nancy. >> My name is Raymond. >> So I think we can eliminate sort of E as an answer choice. >> Can probably also eliminate D, which is also saying that all the nodes are individually assigned and they're not actually pointing to anything else. I think after you add more nodes they actually point to something. So I don't think D should be the answer. >> Yeah. I think Nancy is definitely correct, and I'm seeing her theory also applies to layer C, cuz it also does not has the next reference to node 1 from node 2. So I mean just for letter a, I mean it's obviously incorrect that the only reference has two values. The next reference of node 0 has one reference to node 1, another reference to node 2, but yeah, do you think it's correct? >> Well just by looking at the constructors, we can see that you can't have two references coming out of the same node. So I think we can eliminate that answer also. >> So let's think through what the learners talked about in their discussion and walk through the solution by drawing the diagram one line at a time. And so we start off our diagram by thinking about what happens when we declare the variable n0. It's of type SLLNode and remember that those angle brackets say that our nodes data will be of type integer. Now, when we wanna create the object we use the new operator and we say we're creating an object when we're using the No-arg constructor. So with the No-arg constructor, we're saying that the default values for our instance variables are null. And so in our memory model, we wanna draw our object that's being pointed to by n0, and we fill in those instance variables with null. That's all we have to do for this first line. Now we go ahead to our second line, where we're declaring a new variable, n1. And notice that the constructor that we're using the build the object that's going to be pointed to by n1 now has some arguments. And so we're going to include those in our memory model. We're going to include an argument for n0, which is the previous node. Now, the first line of the two argument constructor calls the one argument constructor. All right, so we're gonna follow through this carefully. The first thing we need to do is call the constructor for an SLL node with input the data. And the data was that argument one, which is the integer that we want to be stored in our object, the node object. And so the first thing that the object that's being created right now does is, it fills in the data value 1, for the instance variable of this object. Now notice that as we're constructing this object we also get a variable, this, that's pointing to the object that's being constructed. And that's gonna be useful for tracing through the execution as we go ahead and go to the next line. Now these next two lines of code are the trickiest part of the constructor. And so we're gonna go through them in a fair amount of detail. What we're doing here is we want to connect the objects that we've created, and so we need to move some arrows around, we need to move some references around, and so it's a little bit tricky. So in the first part, what we're going to do is we're going to set the value of this current object's next object or next node. And the way that we wanna do that is we wanna read off what the previous object pointed to before. And so we look at the value that's stored in prevNode.next. And so we follow the area, we see that we have a box in our memory model for prevNode and we follow that to look at what's its next value. And that's null and so we need to copy that over to this.next. And so we follow the arrow for this, go into the box for next and so we've copied over null. But now what we'd like to do is say the previous object, the previous node, it used to point somewhere, but now we wanna point it to us, because we want to link the previous node to us. And so we need to update the value of prevNode.next with the current object that we're building. And so that means we wanna update it with the arrow that's being pointed to by this, namely the object that we're currently building. All right, we did it. Now we've got yet another object to create. So let's move our work up above and think about what happens when we execute the constructor SLLNode on new input parameters and we're going to point to the newly created object with the variable n2. All right, so as before, we start off by using the one argument constructor to build an object of type SLLNode and fill in the data field in that object. And then, we do the tricky part with the arrows, and so walking through it we look up what the prevNodes next value is. And notice that in this call for the constructor, we're using n0 or the object pointed to by n0 as the same object as pointed to by prevNode. And so when we read off prevNode.next, we need that yellow box up above. And what we wanna do is copy the arrow that was in there to be the new value of this .next, all right. And so now this.next is going to point to the object that's pointed to by n1 as well. All right, so what we've done there, is we've said that the next value after the object that we're creating is what used to be the next value after the prevNode's object. And now what we're going to do is say the prevNode should point to me to the new object. And so what I need to do is say update the value of prevNode.next with a pointer to the object that I'm building. And so that variable prevNode.next now points to the object that I just built, namely the point that object that was pointed to by this. All right, so we've gone through all of the lines in the main method and now let's clean up our picture just a little bit and see what it would look like with the abstract representation that we had. Okay, so that's a little bit easier to follow. And so let's pick out the elements of the linked list that we have in this picture. So the head of the list is that object pointed to by n0. Also notice that it's a sentinel object over here. Even though it's a linked list node, it's got null data. So it doesn't really contain any important information that's going to be part of our list. It doesn't store any data that's used in our list but it tells us where to start, so our sentinel node is that n0. The first node that has real data is n2, so the node after the sentinel node, and then notice that n2 then points to n1. Now I'm using n2 and n1 as shorthands for the objects pointed to by, but basically what we see is we have a linked list that starts with a sentinel node. And then goes to a node that stores 2, and then goes to a node that stores 1. And that is our list. So in the next concept challenge, we'll take this concept just a little bit further, and see if we really understand those two lines of code that played with the arrows, and make sure that the implementation is perfectly clear.
