Topological Sort

Video Demo for Kahn's Algorithm

Algorithm

Steps involved in finding the topological ordering of a DAG:

• Step-1: Compute in-degree (number of incoming edges) for each of the vertex present in the DAG and initialize the count of visited nodes as 0.
• Step-2: Pick all the vertices with in-degree as 0 and add them into a queue (Enqueue operation)
• Step-3: Remove a vertex from the queue (Dequeue operation) and then increment count of visited nodes by 1. Decrease in-degree by 1 for all its neighbouring nodes. If in-degree of a neighbouring node is reduced to zero, then add it to the queue.
• Step 4: Repeat Step 3 until the queue is empty.
• Step 5: If count of visited nodes is not equal to the number of nodes in the graph, then the topological sort is not possible for the given graph.

Step-wise Representation of Kahn's Algorithm

What is indegree and how to find it?

What is meant by indegree?

It is simply the count of number of edges pointed towards the vertex from other vertices.

How to find indegree?

Traverse the array of edges and simply increase the counter of the destination node by 1. Algorithm:
for each node in Nodes
indegree[node] = 0;
for each edge(src,dest) in Edges
indegree[dest]++;
Video Demo for Kahn's