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

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+---
+id: 587d8256367417b2b2512c77
+title: Adjacency List
+challengeType: 1
+forumTopicId: 301620
+---
+
+## Description
+<section id='description'>
+
+Graphs can be represented in different ways. Here we describe one way, which is called an <dfn>adjacency list</dfn>.
+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.
+<blockquote>Node1: Node2, Node3<br>Node2: Node1<br>Node3: Node1</blockquote>
+Above is an undirected graph because <code>Node1</code> is connected to <code>Node2</code> and <code>Node3</code>, and that information is consistent with the connections <code>Node2</code> and <code>Node3</code> show. An adjacency list for a directed graph would mean each row of the list shows direction. If the above was directed, then <code>Node2: Node1</code> would mean there the directed edge is pointing from <code>Node2</code> towards <code>Node1</code>.
+We can represent the undirected graph above as an adjacency list by putting it within a JavaScript object.
+
+```js
+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.
+
+```js
+var undirectedGArr = [
+  [1, 2], // Node1
+  [0],    // Node2
+  [0]     // Node3
+];
+```
+
+</section>
+
+## Instructions
+<section id='instructions'>
+
+Create a social network as an undirected graph with 4 nodes/people named <code>James</code>, <code>Jill</code>, <code>Jenny</code>, and <code>Jeff</code>. There are edges/relationships between James and Jeff, Jill and Jenny, and Jeff and Jenny.
+</section>
+
+## Tests
+<section id='tests'>
+
+```yml
+tests:
+  - text: <code>undirectedAdjList</code> should only contain four nodes.
+    testString: assert(Object.keys(undirectedAdjList).length === 4);
+  - text: There should be an edge between <code>Jeff</code> and <code>James</code>.
+    testString: assert(undirectedAdjList.James.indexOf("Jeff") !== -1 && undirectedAdjList.Jeff.indexOf("James") !== -1);
+  - text: There should be an edge between <code>Jill</code> and <code>Jenny</code>.
+    testString: assert(undirectedAdjList.Jill.indexOf("Jenny") !== -1 && undirectedAdjList.Jill.indexOf("Jenny") !== -1);
+  - text: There should be an edge between <code>Jeff</code> and <code>Jenny</code>.
+    testString: assert(undirectedAdjList.Jeff.indexOf("Jenny") !== -1 && undirectedAdjList.Jenny.indexOf("Jeff") !== -1);
+```
+
+</section>
+
+## Challenge Seed
+<section id='challengeSeed'>
+
+<div id='js-seed'>
+
+```js
+var undirectedAdjList = {};
+```
+
+</div>
+</section>
+
+## Solution
+<section id='solution'>
+
+```js
+var undirectedAdjList = {
+  James: ['Jeff'],
+  Jill: ['Jenny'],
+  Jenny: ['Jill', 'Jeff'],
+  Jeff: ['James', 'Jenny']
+};
+```
+
+</section>