// Approach 1 - Using Min Heap
public Dictionary <Vertex, int> ShortestPathUsingMinHeap(Graph graph, Vertex srcVertex)
{
// Using MinHeap instead of regular queue.
BinaryMinHeap <Vertex> minHeap = new BinaryMinHeap <Vertex>();
// Stores shortest distance from root to every vertex
Dictionary <Vertex, int> distanceDict = new Dictionary <Vertex, int>();
// Fill heap by initializing all vertex with infinite distance from source vertex
foreach (KeyValuePair <long, Vertex> heapItem in graph.Verticies)
{
minHeap.Add(heapItem.Value, int.MaxValue);
}
// Set distance of source vertex to 0
minHeap.DecreaseWeight(srcVertex, 0);
// Put it in Dictionary/Map
distanceDict[srcVertex] = 0;
// Iterate till heap is not empty
while (!minHeap.IsEmpty())
{
// Get the min value from heap node which has vertex and distance of that vertex from source vertex.
BinaryMinHeap <Vertex> .Node heapNode = minHeap.DeQueueMinNode();
Vertex currentVertex = heapNode.key;
// Update shortest distance of current vertex from source vertex
distanceDict[currentVertex] = heapNode.Weight;
// Iterate through all edges of current vertex
foreach (Edge edge in currentVertex.Edges)
{
// Get the adjacent vertex
Vertex adjacent = edge.Vertex1.Equals(currentVertex) ? edge.Vertex2 : edge.Vertex1;
// If heap does not contain adjacent vertex means adjacent vertex already has shortest distance from source vertex
if (!minHeap.Contains(adjacent))
{
continue;
}
// Add distance of current vertex to edge weight to get distance of adjacent vertex from source vertex
// When it goes through current vertex
int newDistance = distanceDict[currentVertex] + edge.Weight;
// See if this above calculated distance is less than current distance stored for adjacent vertex from source vertex
if (minHeap.GetWeightByKey(adjacent) > newDistance)
{
minHeap.DecreaseWeight(adjacent, newDistance);
}
}
}
return(distanceDict);
}
public List <Edge> PrimsMST(Graph graph)
{
BinaryMinHeap <Vertex> minHeap = new BinaryMinHeap <Vertex>();
// Dictionary of verticies to edge which gave minimum weight to this vertex.
Dictionary <Vertex, Edge> vertexToEdgeDictionary = new Dictionary <Vertex, Edge>();
List <Edge> result = new List <Edge>();
// Insert all vertices with infinite value initially.
foreach (KeyValuePair <long, Vertex> vertexPair in graph.Verticies)
{
minHeap.Add(vertexPair.Value, int.MaxValue);
}
// Start from any random vertex
Vertex startVertex = graph.Verticies[0];
// For the start vertex decrease the value in heap, Dictionary to 0
minHeap.DecreaseWeight(startVertex, 0);
// Iterate till heap, Dictionary has elements in it
while (!minHeap.IsEmpty())
{
// Extract min value vertex from heap + Dictionary
Vertex current = minHeap.ExtractMin();
// Get the corresponding edge for this vertex if present and add it to final result.
// This edge wont be present for first vertex.
Edge spanningTreeEdge = vertexToEdgeDictionary[current];
if (spanningTreeEdge != null)
{
result.Add(spanningTreeEdge);
}
foreach (Edge edge in current.Edges)
{
Vertex adjacent = edge.Vertex1.Equals(current) ? edge.Vertex2 : edge.Vertex1;
// Check if adjacent vertex exist in heap + map and weight attached with this vertex is greater than this edge weight
if (minHeap.Contains(adjacent) && minHeap.GetWeightByKey(adjacent) > edge.Weight)
{
// Decrease the value of adjacent vertex to this edge weight.
minHeap.DecreaseWeight(adjacent, edge.Weight);
// Add vertex->edge mapping in the graph.
vertexToEdgeDictionary[adjacent] = edge;
}
}
}
return(result);
}
public static void BinaryMinHeapTest()
{
BinaryMinHeap <String> heap = new BinaryMinHeap <String>();
heap.Add("Sai", 3);
heap.Add("Sri", 4);
heap.Add("Mahi", 8);
heap.Add("Aishu", 10);
heap.Add("SaiSri", 5);
heap.Add("SriMahi", 2);
heap.Add("SriMahiAishu", 1);
heap.DecreaseWeight("Aishu", 1);
heap.PrintHeap();
heap.PrintPositionMap();
}
