Kahn's Topological Sort

graph/kahns_topo_sort.cpp

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+// Including necessary header file
+#include <bits/stdc++.h>
+using namespace std;
+
+// Function to return list containing vertices in
+// Topological order.
+vector<int> topologicalSort(vector<vector<int> >& adj,
+                            int V)
+{
+    // Vector to store indegree of each vertex
+    vector<int> indegree(V);
+    for (int i = 0; i < V; i++) {
+        for (auto it : adj[i]) {
+            indegree[it]++;
+        }
+    }
+
+    // Queue to store vertices with indegree 0
+    queue<int> q;
+    for (int i = 0; i < V; i++) {
+        if (indegree[i] == 0) {
+            q.push(i);
+        }
+    }
+    vector<int> result;
+    while (!q.empty()) {
+        int node = q.front();
+        q.pop();
+        result.push_back(node);
+
+        // Decrease indegree of adjacent vertices as the
+        // current node is in topological order
+        for (auto it : adj[node]) {
+            indegree[it]--;
+
+            // If indegree becomes 0, push it to the queue
+            if (indegree[it] == 0)
+                q.push(it);
+        }
+    }
+
+    // Check for cycle
+    if (result.size() != V) {
+        cout << "Graph contains cycle!" << endl;
+        return {};
+    }
+
+    return result;
+}
+
+int main()
+{
+
+    // Number of nodes
+    int n = 6;
+
+    // Edges
+    vector<vector<int> > edges
+        = { { 0, 1 }, { 1, 2 }, { 2, 3 },
+            { 4, 5 }, { 5, 1 }, { 5, 2 } };
+
+    // Graph represented as an adjacency list
+    vector<vector<int> > adj(n);
+
+    // Constructing adjacency list
+    for (auto i : edges) {
+        adj[i[0]].push_back(i[1]);
+    }
+
+    // Performing topological sort
+    cout << "Topological sorting of the graph: ";
+    vector<int> result = topologicalSort(adj, n);
+
+    // Displaying result
+    for (auto i : result) {
+        cout << i << " ";
+    }
+
+    return 0;
+}