added c++ code of kahn's algorithm

Kahn's algorithm/Kahn's algorithm.cpp

@@ -0,0 +1,68 @@
+#include <bits/stdc++.h>
+#define ll long long int
+#define v vector<int>
+using namespace std;
+int n, m;
+v in_deg;
+queue<int> q;
+v res;
+void BFS(v adj[], int curr)
+{
+    while (!q.empty())
+    {
+        curr = q.front();
+        res.push_back(curr);
+        q.pop();
+        for (int i = 0; i < adj[curr].size(); i++)
+        {
+            int To = adj[curr][i];
+            in_deg[To]--;
+            if (in_deg[To] == 0)
+                q.push(To);
+        }
+    }
+}
+
+void Kahn_algo(v adj[])
+{
+    for (int i = 0; i < n; i++)
+        if (in_deg[i] == 0)
+            q.push(i);
+    BFS(adj, q.front());
+}
+
+int main()
+{
+    freopen("./input.txt", "r", stdin);
+    freopen("../output.txt", "w", stdout);
+    int a, b;
+    cin >> n >> m;
+    v adj[n];
+    for (int i = 0; i < n; i++)
+        in_deg.push_back(0);
+    for (int i = 0; i < m; i++)
+    {
+        cin >> a >> b;
+        adj[a].push_back(b);
+        in_deg[b]++;
+    }
+    Kahn_algo(adj);
+    for (int i = 0; i < res.size(); i++)
+        cout << res[i] << " ";
+
+    return 0;
+}
+/*
+INPUT :
+5
+6
+1 0
+2 1
+2 4
+4 1
+3 0
+3 1
+
+OUTPUT:
+2 3 4 1 0
+*/