C++ Program to Remove the Edges in a Cyclic Graph such that its Linear Extension can be Found

This is a C++ Program to find feedback arc set. This is the set which contains edges which when removed from the graph, graph becomes directed acyclic graph.

Here is source code of the C++ Program to Remove the Edges in a Given Cyclic Graph such that its Linear Extension can be Found. The C++ program is successfully compiled and run on a Linux system. The program output is also shown below.

```
#include<stdio.h>
```
```
#include<iostream>
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```
#include<conio.h>
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```
using namespace std;
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int c = 0;
```
```
struct adj_list
```
```
{
```
```
        int dest;
```
```
        adj_list *next;
```

}*np = NULL, *np1 = NULL, *p = NULL, *q = NULL;

```
struct Graph
```
```
{
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        int v;
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        adj_list *ptr;
```
```
} array[6];
```
```
void addReverseEdge(int src, int dest)
```
```
{
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```
    np1 = new adj_list;
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    np1->dest = src;
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    np1->next = NULL;
```
```
    if (array[dest].ptr == NULL)
```
```
    {
```
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        array[dest].ptr = np1;
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        q = array[dest].ptr;
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        q->next = NULL;
```
```
    }
```
```
    else
```
```
    {
```
```
        q = array[dest].ptr;
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```
        while (q->next != NULL)
```
```
        {
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```
            q = q->next;
```
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        }
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        q->next = np1;
```
```
    }
```
```
}
```
```
void addEdge(int src, int dest)
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```
{
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    np = new adj_list;
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    np->dest = dest;
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    np->next = NULL;
```
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    if (array[src].ptr == NULL)
```
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    {
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        array[src].ptr = np;
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        p = array[src].ptr;
```
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        p->next = NULL;
```
```
    }
```
```
    else
```
```
    {
```
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        p = array[src].ptr;
```
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        while (p->next != NULL)
```
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        {
```
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            p = p->next;
```
```
        }
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        p->next = np;
```
```
    }
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    //addReverseEdge(src, dest);
```
```
}
```
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void print_graph(int n)
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{
```
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    for (int i = 0; i < n; i++)
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```
    {
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        cout << "Adjacency List of " << array[i].v << ": ";

```
        while (array[i].ptr != NULL)
```
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        {
```

            cout << (array[i].ptr)->dest << " ";

            array[i].ptr = (array[i].ptr)->next;

```
        }
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        cout << endl;
```
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    }
```
```
}
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```
 
```
```
int checkDAG(int n)
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{
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    int count = 0;
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    int size = n - 1;
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    for (int i = 0; i < n; i++)
```
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    {
```
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        if (count == size)
```
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        {
```
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            return 0;
```
```
        }
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        if (array[i].ptr == NULL)
```
```
        {
```
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            count++;
```
```
            for (int j = 0; j < n; j++)
```
```
            {
```
```
 
```

                while (array[j].ptr != NULL)

```
                {
```

                    if ((array[j].ptr)->dest == (array[i].ptr)->dest)

```
                    {
```

                        (array[j].ptr)->dest = -1;

```
                    }
```

                    array[i].ptr = (array[i].ptr)->next;

```
                }
```
```
            }
```
```
 
```
```
        }
```
```
    }
```

    cout<<"after checking dag";    int visited[n + 1];

```
    for (int i = 0; i < n; i++)
```
```
    {
```
```
        while (array[i].ptr != NULL)
```
```
        {
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            cout << (array[i].ptr)->dest << " ";

```
            visited[i] = 1;
```
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            for (int j = 0; j < n; j++)
```
```
            {
```
```
 
```

                while (array[j].ptr != NULL)

```
                {
```

                    cout << (array[j].ptr)->dest << " ";

                    if (visited[array[j].v] == 1)

```
                    {
```

                        cout << array[i].v << " - " << array[j].v;

```
                    }
```

                    array[j].ptr = (array[j].ptr)->next;

```
                }
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                cout << endl;
```
```
            }
```
```
 
```

            array[i].ptr = (array[i].ptr)->next;

```
        }
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        cout << endl;
```
```
    }
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    return 1;
```
```
}
```
```
int main()
```
```
{
```
```
    int n = 6;
```

    cout << "Number of vertices: " << n << endl;

```
 
```
```
    for (int i = 0; i < n; i++)
```
```
    {
```
```
        array[i].v = i;
```
```
        array[i].ptr = NULL;
```
```
    }
```
```
    addEdge(0, 1);
```
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    addEdge(1, 2);
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    addEdge(1, 3);
```
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    addEdge(3, 4);
```
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    addEdge(4, 5);
```
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    addEdge(3, 5);
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    addEdge(5, 2);
```
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    print_graph(n);
```
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    cout << "Feedback arc Set: ";
```
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    if (checkDAG(n) == 0)
```
```
        cout << " None";
```
```
}
```

Output:

$ g++ MakeDAG.cpp
$ a.out
 
Number of vertices: 6
Adjacency List of 0: 1 
Adjacency List of 1: 2 3 
Adjacency List of 2: 
Adjacency List of 3: 4 5 
Adjacency List of 4: 5 
Adjacency List of 5: 2 
Feedback arc Set:  None
------------------
(program exited with code: 0)
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