From 85086f246598fa7211650f6ee90fc40d8fbbca04 Mon Sep 17 00:00:00 2001
From: Ellika Mishra <55554508+ellikamishra@users.noreply.github.com>
Date: Fri, 9 Jul 2021 18:55:18 +0530
Subject: [PATCH] chore(CPlusPlus): topological sort on graphs (#381)

Co-authored-by: Ujjwal <75884061+UG-SEP@users.noreply.github.com>
---
 .../CPlusPlus/Graphs/topological-sort.cpp     | 170 ++++++++++++++++++
 algorithms/CPlusPlus/README.md                |   1 +
 2 files changed, 171 insertions(+)
 create mode 100644 algorithms/CPlusPlus/Graphs/topological-sort.cpp

diff --git a/algorithms/CPlusPlus/Graphs/topological-sort.cpp b/algorithms/CPlusPlus/Graphs/topological-sort.cpp
new file mode 100644
index 00000000..2ba5a5ed
--- /dev/null
+++ b/algorithms/CPlusPlus/Graphs/topological-sort.cpp
@@ -0,0 +1,170 @@
+//Topological sort to check order using dfs
+
+#include<bits/stdc++.h>
+
+using namespace std;
+
+/*
+Using graph class with vector of nodes to represent DAG(Directed acyclic graph)
+adding edge and display functions in class along with Cycle to check for cycle
+and if no cycle exits then print a topological sort.
+*/
+class Graph {
+
+  int n = 0;
+  vector < int > * EdgeList;
+
+  public:
+    Graph() {
+      n = 0;
+    }
+
+  Graph(int s) { //initialize edgelist as vector of nodes
+    n = s;
+    EdgeList = new vector < int > [n];
+  }
+  void addEdge(int src, int dest) { //add edges
+
+    EdgeList[src].push_back(dest);
+
+  }
+
+  bool Cycle(int v, vector < int > & visited, vector < int > & rec) { //Check cycle
+
+    if (!visited[v]) {
+
+      visited[v] = 1;
+      rec[v] = 1;
+
+      for (auto it: EdgeList[v]) {
+
+        if (!visited[it] && Cycle(it, visited, rec)) return true;
+
+        else if (rec[it]) return true;
+
+      }
+    }
+
+    rec[v] = 0;
+    return false;
+  }
+
+  void TopSort(int v, vector < int > & visited, stack < int > & s) { //Topological sorted elements added in stack
+
+    visited[v] = 1;
+    vector < int > ::iterator it;
+    for (it = EdgeList[v].begin(); it != EdgeList[v].end(); it++) {
+
+      if (!visited[ * it]) TopSort( * it, visited, s); //visit graph in order(depth first) for topological
+
+    }
+
+    s.push(v);
+
+  }
+
+};
+
+int main() {
+
+  int flag = 0, n, k, src, dest;
+  cout << "\nEnter no. of nodes\n"; //input number of nodes
+  cin >> n;
+  Graph g(n);
+  vector < int > visited(n, 0); //stack to keep track of topological sort,visited vector to keep track of nodes visited and rec stack to detect cycle
+  vector < int > rec(n, 0);
+  stack < int > s;
+
+  for (int i = 0; i < n; i++) {
+    cout << "Enter 1 to add edge from node " << i << ",0 to exit" << "\n"; //take input
+    cin >> k;
+    while (k) {
+
+      cout << "\nEnter dest\n";
+      cin >> dest;
+      g.addEdge(i, dest);
+
+      cout << "Enter 1 to add edge from node " << i << ",0 to exit" << "\n";
+      cin >> k;
+    }
+  }
+
+  for (int i = 0; i < n; i++) {
+    if (g.Cycle(i, visited, rec)) { //check for cycle
+
+      flag = 1;
+      break;
+    }
+  }
+
+  if (flag) cout << "Not DAG"; //if not cycle then find topological sort 
+
+  else {
+
+    fill(visited.begin(), visited.end(), 0);
+
+    for (int i = 0; i < n; i++) {
+      if (!visited[i]) g.TopSort(i, visited, s);
+    }
+    cout << "\nOne topological sorted order: ";
+    while (!s.empty()) { //print stack contents
+
+      cout << s.top() << " ";
+      s.pop();
+    }
+  }
+
+  return 0;
+}
+
+/*
+
+Time Complexity-O(V+E)
+Space Complexity-O(V)
+
+Enter no. of nodes
+6
+Enter 1 to add edge from node 0,0 to exit
+0
+Enter 1 to add edge from node 1,0 to exit
+0
+Enter 1 to add edge from node 2,0 to exit
+1
+
+Enter dest
+3
+Enter 1 to add edge from node 2,0 to exit
+0
+Enter 1 to add edge from node 3,0 to exit
+1
+
+Enter dest
+1
+Enter 1 to add edge from node 3,0 to exit
+0
+Enter 1 to add edge from node 4,0 to exit
+1
+
+Enter dest
+0
+Enter 1 to add edge from node 4,0 to exit
+1
+
+Enter dest
+1
+Enter 1 to add edge from node 4,0 to exit
+0
+Enter 1 to add edge from node 5,0 to exit
+1
+
+Enter dest
+0
+Enter 1 to add edge from node 5,0 to exit
+1
+
+Enter dest
+2
+Enter 1 to add edge from node 5,0 to exit
+0
+One Topological sorted order: 5 4 2 3 1 0
+*/
diff --git a/algorithms/CPlusPlus/README.md b/algorithms/CPlusPlus/README.md
index 083ed512..3ec34a3c 100644
--- a/algorithms/CPlusPlus/README.md
+++ b/algorithms/CPlusPlus/README.md
@@ -17,6 +17,7 @@
 1. [Bellman Ford Algorithm](Graphs/bellmam-ford.cpp)
 2. [kruskal Algorithm](Graphs/kruskal-algorithm.cpp)
 3. [Breadth First Search](Graphs/breadth-first-search.cpp)
+4. [Topological sort](Graphs/topological-sort.cpp)
 
 ## Multiplication