Fix BUG in graph reverse method & Add needed tests

src/data-structures/graph/Graph.js

@@ -144,14 +144,19 @@ export default class Graph {
    */
   reverse() {
     /** @param {GraphEdge} edge */
+    const reversedEdges = [];
     this.getAllEdges().forEach((edge) => {
       // Delete straight edge from graph and from vertices.
       this.deleteEdge(edge);
 
       // Reverse the edge.
       edge.reverse();
 
-      // Add reversed edge back to the graph and its vertices.
+      // Add reversed edge to the list of reversed edges.
+      reversedEdges.push(edge);
+    });
+    reversedEdges.forEach((edge) => {
+      // Add reversed edge to the graph.
       this.addEdge(edge);
     });
 

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

@@ -318,6 +318,35 @@ describe('Graph', () => {
     expect(graph.getNeighbors(vertexD)[0].getKey()).toBe(vertexC.getKey());
   });
 
+  it('should be possible to reverse directed graph with cycle of lenght two', () => {
+    const vertexA = new GraphVertex('A');
+    const vertexB = new GraphVertex('B');
+
+    const edgeAB = new GraphEdge(vertexA, vertexB);
+    const edgeBA = new GraphEdge(vertexB, vertexA);
+
+    const graph = new Graph(true);
+    graph
+      .addEdge(edgeAB)
+      .addEdge(edgeBA);
+
+    expect(graph.toString()).toBe('A,B');
+    expect(graph.getAllEdges().length).toBe(2);
+    expect(graph.getNeighbors(vertexA).length).toBe(1);
+    expect(graph.getNeighbors(vertexA)[0].getKey()).toBe(vertexB.getKey());
+    expect(graph.getNeighbors(vertexB).length).toBe(1);
+    expect(graph.getNeighbors(vertexB)[0].getKey()).toBe(vertexA.getKey());
+
+    graph.reverse();
+
+    expect(graph.toString()).toBe('A,B');
+    expect(graph.getAllEdges().length).toBe(2);
+    expect(graph.getNeighbors(vertexA).length).toBe(1);
+    expect(graph.getNeighbors(vertexA)[0].getKey()).toBe(vertexB.getKey());
+    expect(graph.getNeighbors(vertexB).length).toBe(1);
+    expect(graph.getNeighbors(vertexB)[0].getKey()).toBe(vertexA.getKey());
+  });
+
   it('should return vertices indices', () => {
     const vertexA = new GraphVertex('A');
     const vertexB = new GraphVertex('B');