Implements Khan's topological sorting algorithm as described on wikipedia

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

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+import GraphVertex from '../../../../data-structures/graph/GraphVertex';
+import GraphEdge from '../../../../data-structures/graph/GraphEdge';
+import Graph from '../../../../data-structures/graph/Graph';
+import topologicalSort from '../topologicalSortByCounting';
+
+describe('topologicalSortByCounting', () => {
+  it('should do topological sorting on graph', () => {
+    const vertexNames = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I'];
+    const edges = [
+      { start: 'A', end: 'C' },
+      { start: 'A', end: 'G' },
+      { start: 'A', end: 'I' },
+      { start: 'B', end: 'C' },
+      { start: 'B', end: 'D' },
+      { start: 'C', end: 'E' },
+      { start: 'D', end: 'F' },
+      { start: 'E', end: 'F' },
+      { start: 'E', end: 'H' },
+      { start: 'F', end: 'G' },
+    ];
+    const nameToVertex = {};
+    const graph = new Graph(true);
+
+    vertexNames.forEach((name) => {
+      const vertex = new GraphVertex(name);
+      nameToVertex[name] = vertex;
+    });
+    edges.forEach((edge) => {
+      const startVertex = nameToVertex[edge.start];
+      const endVertex = nameToVertex[edge.end];
+
+      graph.addEdge(new GraphEdge(startVertex, endVertex));
+    });
+
+    const sortedVertices = topologicalSort(graph);
+
+    expect(sortedVertices).toBeDefined();
+    expect(sortedVertices.length).toBe(graph.getAllVertices().length);
+    graph.getAllEdges().forEach((edge) => {
+      expect(sortedVertices.indexOf(edge.startVertex))
+        .toBeLessThan(sortedVertices.indexOf(edge.endVertex));
+    });
+  });
+});

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

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+import PriorityQueue from '../../../data-structures/priority-queue/PriorityQueue';
+
+/**
+ * @param {Graph} graph
+ * Implements Khan's algorithms
+ */
+export default function topologicalSort(graph) {
+  // Count the preceders for all ndoes.
+  const precederCounts = {};
+
+  // Order nodes by number of preceders.
+  const priorityQueue = new PriorityQueue();
+
+  // The resulting sorted list.
+  const sortedStack = [];
+
+  // Initialize the counter array.
+  graph.getAllVertices().forEach((vertex) => {
+    precederCounts[vertex.getKey()] = 0;
+  });
+
+  // Count preceders for all nodes.
+  graph.getAllEdges().forEach((edge) => {
+    precederCounts[edge.endVertex.getKey()] = precederCounts[edge.endVertex.getKey()] + 1;
+  });
+
+  graph.getAllVertices().forEach((vertex) => {
+    priorityQueue.add(vertex, precederCounts[vertex.getKey()]);
+  });
+
+  while (!priorityQueue.isEmpty()) {
+    const vertex = priorityQueue.poll();
+
+    sortedStack.push(vertex);
+    vertex.getEdges().forEach((edge) => {
+      // Decrease counter for all child nodes.
+      precederCounts[edge.endVertex.getKey()] = precederCounts[edge.endVertex.getKey()] - 1;
+
+      // Update the information in priority queue.
+      priorityQueue.changePriority(edge.endVertex, precederCounts[edge.endVertex.getKey()]);
+    });
+  }
+
+  return sortedStack;
+}