freeCodeCamp/curriculum/challenges/english/10-coding-interview-prep/data-structures/adjacency-list.md

id, title, challengeType, forumTopicId, dashedName

id	title	challengeType	forumTopicId	dashedName
587d8256367417b2b2512c77	Adjacency List	1	301620	adjacency-list

--description--

Graphs can be represented in different ways. Here we describe one way, which is called an adjacency list. An adjacency list is essentially a bulleted list where the left side is the node and the right side lists all the other nodes it's connected to. Below is a representation of an adjacency list.

Node1: Node2, Node3
Node2: Node1
Node3: Node1

Above is an undirected graph because Node1 is connected to Node2 and Node3, and that information is consistent with the connections Node2 and Node3 show. An adjacency list for a directed graph would mean each row of the list shows direction. If the above was directed, then Node2: Node1 would mean there the directed edge is pointing from Node2 towards Node1. We can represent the undirected graph above as an adjacency list by putting it within a JavaScript object.

var undirectedG = {
  Node1: ["Node2", "Node3"],
  Node2: ["Node1"],
  Node3: ["Node1"]
};

This can also be more simply represented as an array where the nodes just have numbers rather than string labels.

var undirectedGArr = [
  [1, 2], // Node1
  [0],    // Node2
  [0]     // Node3
];

--instructions--

Create a social network as an undirected graph with 4 nodes/people named James, Jill, Jenny, and Jeff. There are edges/relationships between James and Jeff, Jill and Jenny, and Jeff and Jenny.

--hints--

undirectedAdjList should only contain four nodes.

assert(Object.keys(undirectedAdjList).length === 4);

There should be an edge between Jeff and James.

assert(
  undirectedAdjList.James.indexOf('Jeff') !== -1 &&
    undirectedAdjList.Jeff.indexOf('James') !== -1
);

There should be an edge between Jill and Jenny.

assert(
  undirectedAdjList.Jill.indexOf('Jenny') !== -1 &&
    undirectedAdjList.Jill.indexOf('Jenny') !== -1
);

There should be an edge between Jeff and Jenny.

assert(
  undirectedAdjList.Jeff.indexOf('Jenny') !== -1 &&
    undirectedAdjList.Jenny.indexOf('Jeff') !== -1
);

--seed--

--seed-contents--

var undirectedAdjList = {};

--solutions--

var undirectedAdjList = {
  James: ['Jeff'],
  Jill: ['Jenny'],
  Jenny: ['Jill', 'Jeff'],
  Jeff: ['James', 'Jenny']
};