Fix the findEdge method of the graph (trekhleb#80)

* Fix LinkedList

* Fix the prepend method for the LinkedList

* Fix the remove method for the MinHeap

* Correct a comment

* Fix BST removal method

* Fix the findEdge method of the graph

Author: m-maksyutin

src/data-structures/graph/Graph.js

@@ -115,6 +115,9 @@ export default class Graph {
    */
   findEdge(startVertex, endVertex) {
     const vertex = this.getVertexByKey(startVertex.getKey());
+    if (!vertex) {
+      return null;
+    }
     return vertex.findEdge(endVertex);
   }
 

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

@@ -87,9 +87,11 @@ describe('Graph', () => {
 
     const graphEdgeAB = graph.findEdge(vertexA, vertexB);
     const graphEdgeBA = graph.findEdge(vertexB, vertexA);
-    const graphEdgeAC = graph.findEdge(vertexB, vertexC);
+    const graphEdgeAC = graph.findEdge(vertexA, vertexC);
+    const graphEdgeCA = graph.findEdge(vertexC, vertexA);
 
     expect(graphEdgeAC).toBeNull();
+    expect(graphEdgeCA).toBeNull();
     expect(graphEdgeAB).toEqual(edgeAB);
     expect(graphEdgeBA).toEqual(edgeAB);
     expect(graphEdgeAB.weight).toBe(10);
@@ -108,9 +110,11 @@ describe('Graph', () => {
 
     const graphEdgeAB = graph.findEdge(vertexA, vertexB);
     const graphEdgeBA = graph.findEdge(vertexB, vertexA);
-    const graphEdgeAC = graph.findEdge(vertexB, vertexC);
+    const graphEdgeAC = graph.findEdge(vertexA, vertexC);
+    const graphEdgeCA = graph.findEdge(vertexC, vertexA);
 
     expect(graphEdgeAC).toBeNull();
+    expect(graphEdgeCA).toBeNull();
     expect(graphEdgeBA).toBeNull();
     expect(graphEdgeAB).toEqual(edgeAB);
     expect(graphEdgeAB.weight).toBe(10);