Why a vertex is added to the head of the Linked List in this Adjacency List code?

[ks1]

I am not sure what’s the login behind using insertAtHead instead of insertAtTail ?
Here is the code:

 addEdge(source, destination) {

     if (source < this.vertices && destination < this.vertices)

     //Since we are implementing a directed list, (0,1) is not the same as (1,0)

     this.list[source].insertAtHead(destination);

     //If we were to implement an undirected graph where (0,1)==(1,0),

     //we would create an additional edge from destination to source too:

     //this.list[destination].insertAtHead(source);

   }

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Course: Data Structures for Coding Interviews in JavaScript - Learn Interactively
Lesson: Graph Implementation - Data Structures for Coding Interviews in JavaScript

[Muntha_Amjad]

Hi @ks1
Thanks for reaching out to us. The code in the lesson is just provided for the implementation purpose.

• If we use insertAtHead logic the following Adjacency List is created:
  

  

• If we use insertAtTail logic the following Adjacency List is created:
  

  
  
  So, It makes no difference using insertAtHead or insertAtTail because the order does not matter, the main thing which matter is all the vertices that are pointed by 0 or any other vertex must be present in the linked list either use InsertAtHead or InsertAtTail logic.

Hope it will help, Happy Learning