/// <summary>
/// To solve this since we can start from any cell on the left axis and we can end at any cell on the right axis
/// Lets define a source cell which has edges which connect to all the cells on the left axis ie mat[i,0]
/// Also define a destination cell which has all edges connected to all the cells on the right axis ie mat[i,mat.Length-1]
///
/// Then we can do a djkstra's shortest path algorithm to figure out the shortest path.
/// The running time of this algo is O(V+E) in worse case E = O(V^2)
/// </summary>
/// <param name="mat"></param>
/// <returns></returns>
public List <Cell> GetPath(int[,] mat)
{
UndirectedGraphWithVertexDictionary udg = new UndirectedGraphWithVertexDictionary();
// Fix the source and destination cells.
GraphVertexWithDistance source = new GraphVertexWithDistance(new Cell(-1, -1));
GraphVertexWithDistance destination = new GraphVertexWithDistance(new Cell(mat.Length, mat.Length));
// Create the graph vertices and put it in a matrix
GraphVertexWithDistance[,] allVertex = CreateGraphFromMatrix(mat, source, destination);
// Now we need to do dijkstras shortest path algorithm. Each edge will have a weight of 1.
return(DijkstrasShortestPath(source, destination, allVertex));
}
/// <summary>
/// Create graph from the matrix and add all the edges in the format shown below
/// |------*---*-------*|
/// |--*---*-------*----|
/// |--*------*--------*|
///s ---|------*-----*--*---| --- d
/// |---*-------*----*--|
/// |---*---*----------*|
/// |-----*---*-------*-|
/// |---*----------*---*|
/// </summary>
/// <param name="mat"></param>
/// <param name="source"></param>
/// <param name="destination"></param>
/// <returns></returns>
private GraphVertexWithDistance[,] CreateGraphFromMatrix(int[,] mat, GraphVertexWithDistance source, GraphVertexWithDistance destination)
{
// Create the graph from the matrix
GraphVertexWithDistance[,] allVertex = new GraphVertexWithDistance[mat.GetLength(0), mat.GetLength(1)];
for (int i = 0; i < mat.GetLength(0); i++)
{
for (int j = 0; j < mat.GetLength(1); j++)
{
allVertex[i, j] = new GraphVertexWithDistance(new Cell(i, j));
}
}
// Add all the edges to the graph vertices
for (int i = 0; i < mat.GetLength(0); i++)
{
for (int j = 0; j < mat.GetLength(1); j++)
{
if (j == 0 && mat[i, j] != 1)
{
// This is the left side of the matrix
source.AddEdge(allVertex[i, j]);
allVertex[i, j].AddEdge(source);
}
if (j == mat.GetLength(1) - 1 && mat[i, j] != 1)
{
// This is the right side of the matrix
destination.AddEdge(allVertex[i, j]);
allVertex[i, j].AddEdge(destination);
}
if (j + 1 < mat.GetLength(1) && mat[i, j + 1] != 1 && mat[i, j] != 1)
{
// Add the edge to the right of the graphvertex allVertex[i,j]
allVertex[i, j].AddEdge(allVertex[i, j + 1]);
allVertex[i, j + 1].AddEdge(allVertex[i, j]);
}
if (i + 1 < mat.GetLength(0) && mat[i + 1, j] != 1 && mat[i, j] != 1)
{
// Add the edge below graphVertex allVertex[i,j]
allVertex[i, j].AddEdge(allVertex[i + 1, j]);
allVertex[i + 1, j].AddEdge(allVertex[i, j]);
}
}
}
return(allVertex);
}
public GraphEdge(GraphVertexWithDistance source, GraphVertexWithDistance destination)
{
Source = source;
Destination = destination;
Distance = 1;
}
public void AddEdge(GraphVertexWithDistance neighbour)
{
NeighbourEdges.Add(new GraphEdge(this, neighbour));
}
private List <Cell> DijkstrasShortestPath(GraphVertexWithDistance source, GraphVertexWithDistance destination, GraphVertexWithDistance[,] allVertex)
{
// Initialization
MinHeap <GraphVertexWithDistance> mh = new MinHeap <GraphVertexWithDistance>(allVertex.GetLength(0) * allVertex.GetLength(1) + 2);
Dictionary <GraphVertexWithDistance, GraphVertexWithDistance> backtrack = new Dictionary <GraphVertexWithDistance, GraphVertexWithDistance>();
Dictionary <GraphVertexWithDistance, int> FinalDistance = new Dictionary <GraphVertexWithDistance, int>();
// Add all the graphvertices to the minheap
source.Distance = 0;
mh.Insert(source);
mh.Insert(destination);
for (int i = 0; i < allVertex.GetLength(0); i++)
{
for (int j = 0; j < allVertex.GetLength(1); j++)
{
mh.Insert(allVertex[i, j]);
}
}
while (mh.HeapSize > 0)
{
GraphVertexWithDistance vertex = mh.ExtractMin();
FinalDistance[vertex] = vertex.Distance;
if (vertex == destination)
{
// We have reached the destination. No need to calculate final distance for all the other graph vertex
break;
}
foreach (GraphEdge neighbourEdge in vertex.NeighbourEdges)
{
if (!FinalDistance.ContainsKey(neighbourEdge.Destination))
{
if (vertex.Distance + 1 < neighbourEdge.Destination.Distance)
{
GraphVertexWithDistance oldVertex = neighbourEdge.Destination;
neighbourEdge.Destination.Distance = vertex.Distance + 1;
GraphVertexWithDistance newVertex = neighbourEdge.Destination;
mh.Replace(newVertex, oldVertex);
backtrack[neighbourEdge.Destination] = vertex;
}
}
}
}
// Back track to get the path
List <Cell> path = new List <Cell>();
GraphVertexWithDistance currentVertex = destination;
while (currentVertex != source)
{
if (backtrack.ContainsKey(currentVertex))
{
if (currentVertex != destination)
{
path.Add(currentVertex.Id);
}
currentVertex = backtrack[currentVertex];
}
else
{
break;
}
}
path.Reverse();
return(path);
}