Topological sorting for **D**irected **A**cyclic **G**raph (DAG) is a linear ordering of vertices such that for every directed edge uv, vertex u comes before v in the ordering. Topological Sorting for a graph is not possible if the graph is not a DAG.

For example, a topological sorting of the following graph is “5 4 2 3 1 0?. There can be more than one topological sorting for a graph. For example, another topological sorting of the following graph is “4 5 2 0 3 1″. The first vertex in topological sorting is always a vertex with in-degree as 0 (a vertex with no in-coming edges).

A DFS based solution to find a topological sort has already been discussed.

In this article we will see another way to find the linear ordering of vertices in a directed acyclic graph (DAG). The approach is based on the below fact :

**A DAG G has at least one vertex with in-degree 0 and one vertex with out-degree 0**.

**Proof:** There’s a simple proof to the above fact is that a DAG does not contain a cycle which means that all paths will be of finite length. Now let S be the longest path from u(source) to v(destination). Since S is the longest path there can be no incoming edge to u and no outgoing edge from v, if this situation had occurred then S would not have been the longest path

=> indegree(u) = 0 and outdegree(v) = 0

**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 neighboring nodes.
- If in-degree of a neighboring nodes is reduced to zero, then add it to the queue.

**Step 5:** 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.

**How to find in-degree of each node?**

There are 2 ways to calculate in-degree of every vertex:

Take an in-degree array which will keep track of

**1)** Traverse the array of edges and simply increase the counter of the destination node by 1.

for each node in Nodes indegree[node] = 0; for each edge(src,dest) in Edges indegree[dest]++

Time Complexity: O(V+E)

**2)** Traverse the list for every node and then increment the in-degree of all the nodes connected to it by 1.

for each node in Nodes If (list[node].size()!=0) then for each dest in list indegree[dest]++;

Time Complexity: The outer for loop will be executed V number of times and the inner for loop will be executed E number of times, Thus overall time complexity is O(V+E).

The overall time complexity of the algorithm is O(V+E)

Below is C++ implementation of above algorithm. The implementation uses method 2 discussed above for finding indegrees.

## C++

// A C++ program to print topological sorting of a graph // using indegrees. #include<bits/stdc++.h> using namespace std; // Class to represent a graph class Graph { int V; // No. of vertices' // Pointer to an array containing adjacency listsList list<int> *adj; public: Graph(int V); // Constructor // function to add an edge to graph void addEdge(int u, int v); // prints a Topological Sort of the complete graph void topologicalSort(); }; Graph::Graph(int V) { this->V = V; adj = new list<int>[V]; } void Graph::addEdge(int u, int v) { adj[u].push_back(v); } // The function to do Topological Sort. void Graph::topologicalSort() { // Create a vector to store indegrees of all // vertices. Initialize all indegrees as 0. vector<int> in_degree(V, 0); // Traverse adjacency lists to fill indegrees of // vertices. This step takes O(V+E) time for (int u=0; u<V; u++) { list<int>::iterator itr; for (itr = adj[u].begin(); itr != adj[u].end(); itr++) in_degree[*itr]++; } // Create an queue and enqueue all vertices with // indegree 0 queue<int> q; for (int i = 0; i < V; i++) if (in_degree[i] == 0) q.push(i); // Initialize count of visited vertices int cnt = 0; // Create a vector to store result (A topological // ordering of the vertices) vector <int> top_order; // One by one dequeue vertices from queue and enqueue // adjacents if indegree of adjacent becomes 0 while (!q.empty()) { // Extract front of queue (or perform dequeue) // and add it to topological order int u = q.front(); q.pop(); top_order.push_back(u); // Iterate through all its neighbouring nodes // of dequeued node u and decrease their in-degree // by 1 list<int>::iterator itr; for (itr = adj[u].begin(); itr != adj[u].end(); itr++) // If in-degree becomes zero, add it to queue if (--in_degree[*itr] == 0) q.push(*itr); cnt++; } // Check if there was a cycle if (cnt != V) { cout << "There exists a cycle in the graph\n"; return; } // Print topological order for (int i=0; i<top_order.size(); i++) cout << top_order[i] << " "; cout << endl; } // Driver program to test above functions int main() { // Create a graph given in the above diagram Graph g(6); g.addEdge(5, 2); g.addEdge(5, 0); g.addEdge(4, 0); g.addEdge(4, 1); g.addEdge(2, 3); g.addEdge(3, 1); cout << "Following is a Topological Sort of\n"; g.topologicalSort(); return 0; }

## Java

// A Java program to print topological sorting of a graph // using indegrees import java.util.*; //Class to represent a graph class Graph { int V;// No. of vertices //An Array of List which contains //references to the Adjacency List of //each vertex List <Integer> adj[]; public Graph(int V)// Constructor { this.V = V; adj = new ArrayList[V]; for(int i = 0; i < V; i++) adj[i]=new ArrayList<Integer>(); } // function to add an edge to graph public void addEdge(int u,int v) { adj[u].add(v); } // prints a Topological Sort of the complete graph public void topologicalSort() { // Create a array to store indegrees of all // vertices. Initialize all indegrees as 0. int indegree[] = new int[V]; // Traverse adjacency lists to fill indegrees of // vertices. This step takes O(V+E) time for(int i = 0; i < V; i++) { ArrayList<Integer> temp = (ArrayList<Integer>) adj[i]; for(int node : temp) { indegree[node]++; } } // Create a queue and enqueue all vertices with // indegree 0 Queue<Integer> q = new LinkedList<Integer>(); for(int i = 0;i < V; i++) { if(indegree[i]==0) q.add(i); } // Initialize count of visited vertices int cnt = 0; // Create a vector to store result (A topological // ordering of the vertices) Vector <Integer> topOrder=new Vector<Integer>(); while(!q.isEmpty()) { // Extract front of queue (or perform dequeue) // and add it to topological order int u=q.poll(); topOrder.add(u); // Iterate through all its neighbouring nodes // of dequeued node u and decrease their in-degree // by 1 for(int node : adj[u]) { // If in-degree becomes zero, add it to queue if(--indegree[node] == 0) q.add(node); } cnt++; } // Check if there was a cycle if(cnt != V) { System.out.println("There exists a cycle in the graph"); return ; } // Print topological order for(int i : topOrder) { System.out.print(i+" "); } } } // Driver program to test above functions class Main { public static void main(String args[]) { // Create a graph given in the above diagram Graph g=new Graph(6); g.addEdge(5, 2); g.addEdge(5, 0); g.addEdge(4, 0); g.addEdge(4, 1); g.addEdge(2, 3); g.addEdge(3, 1); System.out.println("Following is a Topological Sort"); g.topologicalSort(); } }

## Python

# A Python program to print topological sorting of a graph # using indegrees from collections import defaultdict #Class to represent a graph class Graph: def __init__(self,vertices): self.graph = defaultdict(list) #dictionary containing adjacency List self.V = vertices #No. of vertices # function to add an edge to graph def addEdge(self,u,v): self.graph[u].append(v) # The function to do Topological Sort. def topologicalSort(self): # Create a vector to store indegrees of all # vertices. Initialize all indegrees as 0. in_degree = [0]*(self.V) # Traverse adjacency lists to fill indegrees of # vertices. This step takes O(V+E) time for i in self.graph: for j in self.graph[i]: in_degree[j] += 1 # Create an queue and enqueue all vertices with # indegree 0 queue = [] for i in range(self.V): if in_degree[i] == 0: queue.append(i) #Initialize count of visited vertices cnt = 0 # Create a vector to store result (A topological # ordering of the vertices) top_order = [] # One by one dequeue vertices from queue and enqueue # adjacents if indegree of adjacent becomes 0 while queue: # Extract front of queue (or perform dequeue) # and add it to topological order u = queue.pop(0) top_order.append(u) # Iterate through all neighbouring nodes # of dequeued node u and decrease their in-degree # by 1 for i in self.graph[u]: in_degree[i] -= 1 # If in-degree becomes zero, add it to queue if in_degree[i] == 0: queue.append(i) cnt += 1 # Check if there was a cycle if cnt != self.V: print "There exists a cycle in the graph" else : #Print topological order print top_order g= Graph(6) g.addEdge(5, 2); g.addEdge(5, 0); g.addEdge(4, 0); g.addEdge(4, 1); g.addEdge(2, 3); g.addEdge(3, 1); print "Following is a Topological Sort of the given graph" g.topologicalSort() #This code is contributed by Neelam Yadav

Output :

Following is a Topological Sort 4 5 2 0 3 1

This article is contributed by **Chirag Agarwal**. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above