Working with Graphs & Trees

What type of graph representation is implemented in this code?

class Graph {
  constructor() {
    this.adjacencyList = {};
  }
  
  addVertex(vertex) {
    if (!this.adjacencyList[vertex]) {
      this.adjacencyList[vertex] = [];
    }
  }
  
  addEdge(vertex1, vertex2) {
    this.adjacencyList[vertex1].push(vertex2);
    this.adjacencyList[vertex2].push(vertex1);
  }
}

Edge List
Adjacency Matrix
Adjacency List
This implements an adjacency list representation because: 1) Uses a hash table to store vertices as keys, 2) Each vertex maps to an array of its adjacent vertices, 3) Provides efficient storage for sparse graphs, 4) Enables O(1) vertex addition, 5) Edge lookup is O(degree) where degree is number of adjacent vertices, 6) Memory usage is O(V + E) where V is vertices and E is edges, 7) Common implementation choice for real-world graphs, 8) Suitable for both directed and undirected graphs.
Incidence Matrix

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