Add detect cycle.

src/data-structures/graph/Graph.js

@@ -4,6 +4,7 @@ export default class Graph {
    */
   constructor(isDirected = false) {
     this.vertices = {};
+    this.edges = {};
     this.isDirected = isDirected;
   }
 
@@ -39,6 +40,13 @@ export default class Graph {
     return Object.values(this.vertices);
   }
 
+  /**
+   * @return {GraphEdge[]}
+   */
+  getAllEdges() {
+    return Object.values(this.edges);
+  }
+
   /**
    * @param {GraphEdge} edge
    * @returns {Graph}
@@ -60,7 +68,12 @@ export default class Graph {
       endVertex = this.getVertexByKey(edge.endVertex.getKey());
     }
 
-    // @TODO: Check if edge has been already added.
+    // Check if edge has been already added.
+    if (this.edges[edge.getKey()]) {
+      throw new Error('Edge has already been added before');
+    } else {
+      this.edges[edge.getKey()] = edge;
+    }
 
     // Add edge to the vertices.
     if (this.isDirected) {

src/data-structures/graph/GraphEdge.js

@@ -9,4 +9,14 @@ export default class GraphEdge {
     this.endVertex = endVertex;
     this.weight = weight;
   }
+
+  /**
+   * @return {string}
+   */
+  getKey() {
+    const startVertexKey = this.startVertex.getKey();
+    const endVertexKey = this.endVertex.getKey();
+
+    return `${startVertexKey}_${endVertexKey}`;
+  }
 }

src/data-structures/graph/__test__/Graph.test.js

@@ -136,4 +136,42 @@ describe('Graph', () => {
     expect(neighbors[0]).toEqual(vertexB);
     expect(neighbors[1]).toEqual(vertexC);
   });
+
+  it('should throw an error when trying to add edge twice', () => {
+    function addSameEdgeTwice() {
+      const graph = new Graph(true);
+
+      const vertexA = new GraphVertex('A');
+      const vertexB = new GraphVertex('B');
+
+      const edgeAB = new GraphEdge(vertexA, vertexB);
+
+      graph
+        .addEdge(edgeAB)
+        .addEdge(edgeAB);
+    }
+
+    expect(addSameEdgeTwice).toThrow();
+  });
+
+  it('should return the list of all added edges', () => {
+    const graph = new Graph(true);
+
+    const vertexA = new GraphVertex('A');
+    const vertexB = new GraphVertex('B');
+    const vertexC = new GraphVertex('C');
+
+    const edgeAB = new GraphEdge(vertexA, vertexB);
+    const edgeBC = new GraphEdge(vertexB, vertexC);
+
+    graph
+      .addEdge(edgeAB)
+      .addEdge(edgeBC);
+
+    const edges = graph.getAllEdges();
+
+    expect(edges.length).toBe(2);
+    expect(edges[0]).toEqual(edgeAB);
+    expect(edges[1]).toEqual(edgeBC);
+  });
 });

src/data-structures/graph/__test__/GraphEdge.test.js

@@ -7,6 +7,7 @@ describe('GraphEdge', () => {
     const endVertex = new GraphVertex('B');
     const edge = new GraphEdge(startVertex, endVertex);
 
+    expect(edge.getKey()).toBe('A_B');
     expect(edge.startVertex).toEqual(startVertex);
     expect(edge.endVertex).toEqual(endVertex);
     expect(edge.weight).toEqual(1);