add topological sorting

Author: ShyrenMore

Graph/topological_sort.cpp

@@ -0,0 +1,44 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+/*
+Time Complexity: O(V+E)+O(V), where V = no. of nodes and E = no. of edges. There can be at most V components. So, another O(V) time complexity.
+
+Space Complexity: O(2N) + O(N) ~ O(2N): O(2N) for the visited array and the stack carried during DFS calls and O(N) for recursive stack space, where N = no. of nodes.
+
+https://youtu.be/5lZ0iJMrUMk
+*/
+
+class Solution{
+private:
+    void dfs(int node, vector<int> &visited, stack<int> &st, vector<int> adj[])
+    {
+        visited[node] = 1;
+        for(int neighbour: adj[node])
+        {
+            if(!visited[neighbour])
+                dfs(neighbour, visited, st, adj);
+        }
+
+        st.push(node);
+    }
+
+public:
+    vector<int> topoSort(int V, vector<int> adj[])
+    {
+        vector<int> visited(V, 0);
+        stack<int> st;
+        for (int i = 0; i < V; i++)
+            if(!visited[i])
+                dfs(i, visited, st, adj);
+
+        vector<int> ans;
+        while(!st.empty())
+        {
+            ans.push_back(st.top());
+            st.pop();
+        }
+
+        return ans;
+    }
+};