adjacencyList.js

const vertices = ['james', 'jon', 'joe', 'moe', 'moda'];
const vertices2 = ['jon', 'joe', 'moe'];
const vertices3 = ['james', 'joe', 'moda'];

const edges = [['james', 'jon'], ['james', 'joe'], ['jon', 'joe'], ['joe', 'moe'], ['moe', 'moda'], ['moda', 'james']];
const edges2 = [['jon','joe'], ['joe', 'moe'], ['moe', 'jon']];
const edges3 = [['james', 'joe'], ['moda', 'james']];


function createAdjacencyList(vertices, edges) {
  const adjList = {};
  vertices.forEach(vertex => {
    adjList[vertex] = [];
    edges.forEach(edge => {
      const vertexIndex = edge.indexOf(vertex);
      if (vertexIndex > -1) {
        const adjVertex = edge[Number(!vertexIndex)];
        adjList[vertex].push(adjVertex);
      }
    });
  });

  return adjList;
}

function isNeighbour(adjList, vertex, neighbour) {
  return adjList[vertex].includes(neighbour);
}

function isTree(adjList) {
  // TODO: make this function correct.
  const numVertices = Object.keys(adjList).length;
  let numEdges = 0;
  for (let key in adjList) {
    console.log(adjList[key]);
    // numVertices minus 1 because doesn't include reference to self.
    if (adjList[key].length === numVertices - 1) {
      numEdges++;
    }
  }
  return numEdges !== numVertices;;
}

const adjList = createAdjacencyList(vertices, edges);
const adjList2 = createAdjacencyList(vertices2, edges2);
const adjList3 = createAdjacencyList(vertices3, edges3);

console.log(adjList);
console.log(isTree(adjList));