Add topological sorting.

README.md

@@ -71,7 +71,7 @@
   * [Detect Cycle](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/graph/detect-cycle) - for both directed and undirected graphs (DFS and Disjoint Set based versions)
   * [Prim’s Algorithm](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/graph/prim) - finding Minimum Spanning Tree (MST) for weighted undirected graph
   * [Kruskal’s Algorithm](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/graph/kruskal) - finding Minimum Spanning Tree (MST) for weighted undirected graph
-  * Topological Sorting
+  * [Topological Sorting](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/graph/topological-sorting) - DFS method
   * Eulerian path, Eulerian circuit
   * Strongly Connected Component algorithm
   * Shortest Path Faster Algorithm (SPFA)

src/algorithms/graph/topological-sorting/README.md

@@ -0,0 +1,55 @@
+# Topological Sorting
+
+In the field of computer science, a topological sort or 
+topological ordering of a directed graph is a linear ordering 
+of its vertices such that for every directed edge `uv` from 
+vertex `u` to vertex `v`, `u` comes before `v` in the ordering.
+
+For instance, the vertices of the graph may represent tasks to 
+be performed, and the edges may represent constraints that one 
+task must be performed before another; in this application, a 
+topological ordering is just a valid sequence for the tasks.
+
+A topological ordering is possible if and only if the graph has
+no directed cycles, that is, if it is a [directed acyclic graph](https://en.wikipedia.org/wiki/Directed_acyclic_graph) 
+(DAG). Any DAG has at least one topological ordering, and algorithms are 
+known for constructing a topological ordering of any DAG in linear time.
+
+![Directed Acyclic Graph](https://upload.wikimedia.org/wikipedia/commons/c/c6/Topological_Ordering.svg)
+
+A topological ordering of a directed acyclic graph: every edge goes from 
+earlier in the ordering (upper left) to later in the ordering (lower right). 
+A directed graph is acyclic if and only if it has a topological ordering.
+
+## Example
+
+![Topologic Sorting](https://upload.wikimedia.org/wikipedia/commons/0/03/Directed_acyclic_graph_2.svg)
+
+The graph shown above has many valid topological sorts, including:
+
+- `5, 7, 3, 11, 8, 2, 9, 10` (visual left-to-right, top-to-bottom)
+- `3, 5, 7, 8, 11, 2, 9, 10` (smallest-numbered available vertex first)
+- `5, 7, 3, 8, 11, 10, 9, 2` (fewest edges first)
+- `7, 5, 11, 3, 10, 8, 9, 2` (largest-numbered available vertex first)
+- `5, 7, 11, 2, 3, 8, 9, 10` (attempting top-to-bottom, left-to-right)
+- `3, 7, 8, 5, 11, 10, 2, 9` (arbitrary)
+
+## Application
+
+The canonical application of topological sorting is in 
+**scheduling a sequence of jobs** or tasks based on their dependencies. The jobs 
+are represented by vertices, and there is an edge from `x` to `y` if 
+job `x` must be completed before job `y` can be started (for 
+example, when washing clothes, the washing machine must finish 
+before we put the clothes in the dryer). Then, a topological sort 
+gives an order in which to perform the jobs.
+
+Other application is **dependency resolution**. Each vertex is a package
+and each edge is a dependency of package `a` on package 'b'. Then topological
+sorting will provide a sequence of installing dependencies in a way that every
+next dependency has its dependent packages to be installed in prior.
+
+## References
+
+- [Wikipedia](https://en.wikipedia.org/wiki/Topological_sorting)
+- [Topological Sorting on YouTube by Tushar Roy](https://www.youtube.com/watch?v=ddTC4Zovtbc)

src/algorithms/graph/topological-sorting/__test__/topologicalSort.test.js

@@ -0,0 +1,53 @@
+import GraphVertex from '../../../../data-structures/graph/GraphVertex';
+import GraphEdge from '../../../../data-structures/graph/GraphEdge';
+import Graph from '../../../../data-structures/graph/Graph';
+import topologicalSort from '../topologicalSort';
+
+describe('topologicalSort', () => {
+  it('should do topological sorting on graph', () => {
+    const vertexA = new GraphVertex('A');
+    const vertexB = new GraphVertex('B');
+    const vertexC = new GraphVertex('C');
+    const vertexD = new GraphVertex('D');
+    const vertexE = new GraphVertex('E');
+    const vertexF = new GraphVertex('F');
+    const vertexG = new GraphVertex('G');
+    const vertexH = new GraphVertex('H');
+
+    const edgeAC = new GraphEdge(vertexA, vertexC);
+    const edgeBC = new GraphEdge(vertexB, vertexC);
+    const edgeBD = new GraphEdge(vertexB, vertexD);
+    const edgeCE = new GraphEdge(vertexC, vertexE);
+    const edgeDF = new GraphEdge(vertexD, vertexF);
+    const edgeEF = new GraphEdge(vertexE, vertexF);
+    const edgeEH = new GraphEdge(vertexE, vertexH);
+    const edgeFG = new GraphEdge(vertexF, vertexG);
+
+    const graph = new Graph(true);
+
+    graph
+      .addEdge(edgeAC)
+      .addEdge(edgeBC)
+      .addEdge(edgeBD)
+      .addEdge(edgeCE)
+      .addEdge(edgeDF)
+      .addEdge(edgeEF)
+      .addEdge(edgeEH)
+      .addEdge(edgeFG);
+
+    const sortedVertices = topologicalSort(graph);
+
+    expect(sortedVertices).toBeDefined();
+    expect(sortedVertices.length).toBe(graph.getAllVertices().length);
+    expect(sortedVertices).toEqual([
+      vertexB,
+      vertexD,
+      vertexA,
+      vertexC,
+      vertexE,
+      vertexH,
+      vertexF,
+      vertexG,
+    ]);
+  });
+});

src/algorithms/graph/topological-sorting/topologicalSort.js

@@ -0,0 +1,47 @@
+import Stack from '../../../data-structures/stack/Stack';
+import depthFirstSearch from '../depth-first-search/depthFirstSearch';
+
+/**
+ * @param {Graph} graph
+ */
+export default function topologicalSort(graph) {
+  // Create a set of all vertices we want to visit.
+  const unvisitedSet = {};
+  graph.getAllVertices().forEach((vertex) => {
+    unvisitedSet[vertex.getKey()] = vertex;
+  });
+
+  // Create a set for all vertices that we've already visited.
+  const visitedSet = {};
+
+  // Create a stack of already ordered vertices.
+  const sortedStack = new Stack();
+
+  const dfsCallbacks = {
+    enterVertex: ({ currentVertex }) => {
+      // Add vertex to visited set in case if all its children has been explored.
+      visitedSet[currentVertex.getKey()] = currentVertex;
+
+      // Remove this vertex from unvisited set.
+      delete unvisitedSet[currentVertex.getKey()];
+    },
+    leaveVertex: ({ currentVertex }) => {
+      // If the vertex has been totally explored then we may push it to stack.
+      sortedStack.push(currentVertex);
+    },
+    allowTraversal: ({ nextVertex }) => {
+      return !visitedSet[nextVertex.getKey()];
+    },
+  };
+
+  // Let's go and do DFS for all unvisited nodes.
+  while (Object.keys(unvisitedSet).length) {
+    const currentVertexKey = Object.keys(unvisitedSet)[0];
+    const currentVertex = unvisitedSet[currentVertexKey];
+
+    // Do DFS for current node.
+    depthFirstSearch(graph, currentVertex, dfsCallbacks);
+  }
+
+  return sortedStack.toArray();
+}

src/data-structures/linked-list/LinkedList.js

@@ -9,6 +9,10 @@ export default class LinkedList {
     this.tail = null;
   }
 
+  /**
+   * @param {*} value
+   * @return {LinkedList}
+   */
   prepend(value) {
     // Make new node to be a head.
     this.head = new LinkedListNode(value, this.head);
@@ -16,6 +20,10 @@ export default class LinkedList {
     return this;
   }
 
+  /**
+   * @param {*} value
+   * @return {LinkedList}
+   */
   append(value) {
     const newNode = new LinkedListNode(value);
 
@@ -34,6 +42,10 @@ export default class LinkedList {
     return this;
   }
 
+  /**
+   * @param {*} value
+   * @return {LinkedListNode}
+   */
   delete(value) {
     if (!this.head) {
       return null;
@@ -67,6 +79,12 @@ export default class LinkedList {
     return deletedNode;
   }
 
+  /**
+   * @param {Object} findParams
+   * @param {*} findParams.value
+   * @param {function} [findParams.callback]
+   * @return {LinkedListNode}
+   */
   find({ value = undefined, callback = undefined }) {
     if (!this.head) {
       return null;
@@ -91,6 +109,9 @@ export default class LinkedList {
     return null;
   }
 
+  /**
+   * @return {LinkedListNode}
+   */
   deleteTail() {
     if (this.head === this.tail) {
       const deletedTail = this.tail;
@@ -116,6 +137,9 @@ export default class LinkedList {
     return deletedTail;
   }
 
+  /**
+   * @return {LinkedListNode}
+   */
   deleteHead() {
     if (!this.head) {
       return null;
@@ -133,6 +157,9 @@ export default class LinkedList {
     return deletedHead;
   }
 
+  /**
+   * @return {LinkedListNode[]}
+   */
   toArray() {
     const nodes = [];
 
@@ -145,6 +172,10 @@ export default class LinkedList {
     return nodes;
   }
 
+  /**
+   * @param {function} [callback]
+   * @return {string}
+   */
   toString(callback) {
     return this.toArray().map(node => node.toString(callback)).toString();
   }

src/data-structures/stack/Stack.js

@@ -5,10 +5,16 @@ export default class Stack {
     this.linkedList = new LinkedList();
   }
 
+  /**
+   * @return {boolean}
+   */
   isEmpty() {
     return !this.linkedList.tail;
   }
 
+  /**
+   * @return {LinkedListNode}
+   */
   peek() {
     if (!this.linkedList.tail) {
       return null;
@@ -17,15 +23,35 @@ export default class Stack {
     return this.linkedList.tail.value;
   }
 
+  /**
+   * @param {*} value
+   */
   push(value) {
     this.linkedList.append(value);
   }
 
+  /**
+   * @return {LinkedListNode}
+   */
   pop() {
     const removedTail = this.linkedList.deleteTail();
     return removedTail ? removedTail.value : null;
   }
 
+  /**
+   * @return {*[]}
+   */
+  toArray() {
+    return this.linkedList
+      .toArray()
+      .map(linkedListNode => linkedListNode.value)
+      .reverse();
+  }
+
+  /**
+   * @param {function} [callback]
+   * @return {string}
+   */
   toString(callback) {
     return this.linkedList.toString(callback);
   }

src/data-structures/stack/__test__/Stack.test.js

@@ -62,4 +62,16 @@ describe('Stack', () => {
     expect(stack.pop().value).toBe('test2');
     expect(stack.pop().value).toBe('test1');
   });
+
+  it('should be possible to convert stack to array', () => {
+    const stack = new Stack();
+
+    expect(stack.peek()).toBeNull();
+
+    stack.push(1);
+    stack.push(2);
+    stack.push(3);
+
+    expect(stack.toArray()).toEqual([3, 2, 1]);
+  });
 });