Create topological_sort_by_kahns_algo.cpp (#778)

* Create topological_sort_by_kahns_algo.cpp

implementation of topological sorting by kahn's algo

* Update topological_sort_by_kahns_algo

put c system headers before c++ system headers

* Update  topological_sort_by_kahns_algo.cpp

improves syntax (tabs/whitespaces etc)

* Update topological_sort_by_kahns_algo.cpp

removed errors

* Update graph/topological_sort_by_kahns_algo.cpp

improved syntax

Co-authored-by: Christian Clauss <cclauss@me.com>

* Fix indentation

* Fix include order

Co-authored-by: Christian Clauss <cclauss@me.com>

graph/topological_sort_by_kahns_algo.cpp

@@ -0,0 +1,68 @@
+#include <stdio.h>
+#include <string.h>
+#include <iostream>
+#include <vector>
+#include <queue>
+
+int *topoSortKahn(int N, std::vector<int> adj[]);
+
+int main() {
+    int nodes, edges;
+    std::cin >> edges >> nodes;
+    if (edges == 0 || nodes == 0)
+        return 0;
+    int u, v;
+
+    std::vector<int>graph[nodes];
+    // create graph
+    // example
+    // 6 6
+    // 5 0 5 2 2 3 4 0 4 1 1 3
+
+    for (int i = 0; i < edges; i++) {
+        std::cin >> u >> v;
+        graph[u].push_back(v);
+    }
+
+    int *topo = topoSortKahn(nodes, graph);
+    // topologically sorted nodes
+    for (int i = 0; i < nodes; i++) {
+        std::cout << topo[i] << " ";
+    }
+}
+
+int* topoSortKahn(int V, std::vector<int> adj[]) {
+    std::vector<bool>vis(V+1, false);
+    std::vector<int>deg(V+1, 0);
+    for (int i = 0; i < V; i++) {
+        for (int j = 0; j < adj[i].size(); j++) {
+            deg[adj[i][j]]++;
+        }
+    }
+    std::queue<int>q;
+    for (int i = 0; i < V; i++) {
+        if (deg[i] == 0) {
+            q.push(i);
+            vis[i] = true;
+        }
+    }
+    int *arr = new int[V+1];
+    memset(arr, 0, V+1);
+    int count = 0;
+    while (!q.empty()) {
+        int cur = q.front();
+        q.pop();
+        arr[count] = cur;
+        count++;
+        for (int i = 0; i < adj[cur].size(); i++) {
+            if (!vis[adj[cur][i]]) {
+                deg[adj[cur][i]]--;
+                if (deg[adj[cur][i]] == 0) {
+                    q.push(adj[cur][i]);
+                    vis[adj[cur][i]] = true;
+                }
+            }
+        }
+    }
+    return arr;
+}