C++

Forum

 
Detecting Cycle using DFS on specific nodes in graph

tristanbox09 (13)
Hello guys, I am trying to use DFS to see if there is a cycle detected in the graph. I want to input selected nodes and check if the selected nodes form a cycle within the whole graph... so rather than doing a full DFS on the whole graph, I would do it on the selected nodes to see if a cycle appears or not. I Input a graph between nodes 0 - 6 which has various connections (diagram can be seen below)... when I try to see if (2,5,6) are connected or not with my DFS function, it will output that these 3 nodes are connected, but they aren't.
Any ideas on how to fix this?

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
 
  #include<iostream>
#include<list>
#include<vector>
#include<stack>

using namespace std;

class Graph {

private:

    int nodes;
    list<int>* adjlist;
    vector<bool> visited;

    // parent stores the parent node of the visited node, this is used
    // for finding cycle in an un-directed graph.
    vector<int> parent;

    bool cycle_present;

public:
    int count = 0;
    Graph() {
    }

    Graph(int arg_nodes) {

        this->nodes = arg_nodes;
        adjlist = new list<int>[nodes];
        visited.resize(nodes, false);
        parent.resize(nodes, -1);
        cycle_present = false;
    }

    ~Graph() {
        delete[] adjlist;
    }

    void AddEdge(int src, int dst, bool bidirectional) {

        adjlist[src].push_back(dst);

        if (bidirectional)
            adjlist[dst].push_back(src);
    }

    // Function detects cycle in an undirected graph
    //
    void DFS_DetectCycleInUndirectedGraph(int src, vector<int>& lucky) {

        visited[src] = true;

        
        for (auto& adj_node : adjlist[src]) {

            if (!visited[adj_node]) {

                parent[adj_node] = src;
                DFS_DetectCycleInUndirectedGraph(adj_node, lucky);

            }
            else if (parent[src] != adj_node) {

                cycle_present = true;
                return;

            }
        }
    }

    bool IsCyclePresent() {
        return cycle_present;
    }
};

int main()
{
    Graph g1_undirected(7);

    /* Undirected graph 1
               0
              / \
             1 - 2
                / \
               3   4
                  / \
                 5  6 */
    g1_undirected.AddEdge(0, 1, true);
    g1_undirected.AddEdge(0, 2, true);
    g1_undirected.AddEdge(1, 2, true);
    g1_undirected.AddEdge(2, 3, true);
    g1_undirected.AddEdge(2, 4, true);
    g1_undirected.AddEdge(4, 5, true);
    g1_undirected.AddEdge(4, 6, true);

    vector<int> lucky;
    lucky.push_back(2);
    lucky.push_back(5);
    lucky.push_back(6);

    g1_undirected.DFS_DetectCycleInUndirectedGraph(lucky[0], lucky);  // (2, <2,5,6>)

    if (g1_undirected.IsCyclePresent()) {
        cout << "Cycle found in the undirected graph g1" << endl;
    }
    else {
        cout << "Cycle not found in the undirected graph g1" << endl;
    }

    lucky.clear();

    return 0;
}
lastchance (6980)
At the moment you aren't distinguishing nodes that are in "lucky" from those that aren't.

You can change
for (auto& adj_node : adjlist[src]) {
to
1
2
 
        for (auto& adj_node : adjlist[src]) {
            if ( find( lucky.begin(), lucky.end(), adj_node ) == lucky.end() ) continue;


You will need (for find):
#include <algorithm>


A better method would simply be to create a subgraph and run the normal algorithm.
Topic archived. No new replies allowed.