There is an issue in DirectedWeightedGraph.RemoveVertex #368
Description
The current implementation sets the value of the Vertices array to null at the index of the vertex to be removed, and decrements the Count member. This adds a null entry to the array and keeps the index of all other vertices the same. Since Count is decremented, the last element in the array will be overwritten when adding a new vertex.
I made a simple update to the test case to show that the index of the last vertex before the Remove operation will have the same index as a newly added vertex after the Remove operation (Assert.AreNotEqual). The test case fails. This will clearly have consequences since the index is used for all other operations.
[Test]
public void GraphRemoveVertexTest_Success()
{
var graph = new DirectedWeightedGraph<char>(10);
var vertexA = graph.AddVertex('A');
var vertexB = graph.AddVertex('B');
var vertexC = graph.AddVertex('C');
graph.AddEdge(vertexB, vertexA, 5);
graph.AddEdge(vertexC, vertexA, 5);
var neighborsB = graph.GetNeighbors(vertexB).ToList();
var neighborsC = graph.GetNeighbors(vertexC).ToList();
graph.RemoveVertex(vertexA);
var vertexD = graph.AddVertex('D');
Assert.AreNotEqual(vertexC.Index, vertexD.Index);
neighborsB.Should().HaveCount(1);
neighborsB[0].Should().Be(vertexA);
neighborsC.Should().HaveCount(1);
neighborsC[0].Should().Be(vertexA);
graph.GetNeighbors(vertexB).Should().HaveCount(0);
graph.GetNeighbors(vertexC).Should().HaveCount(0);
}