detect_cycle_graph.cpp

#include<iostream>
#include<set>
#define NODE 5
using namespace std;

int graph[NODE][NODE] = {
   {0, 1, 0, 0, 0},
   {1, 0, 1, 1, 0},
   {0, 1, 0, 0, 1},
   {0, 1, 0, 0, 1},
   {0, 0, 1, 1, 0}
};

bool dfs(int vertex, set<int>&visited, int parent) {
   visited.insert(vertex);
   for(int v = 0; v<NODE; v++) {
      if(graph[vertex][v]) {
         if(v == parent)    //if v is the parent not move that direction
            continue;
         if(visited.find(v) != visited.end())    //if v is already visited
            return true;
         if(dfs(v, visited, vertex))
            return true;
      }
   }
   return false;
}

bool hasCycle() {
   set<int> visited;       //visited set
   for(int v = 0; v<NODE; v++) {
      if(visited.find(v) != visited.end())    //when visited holds v, jump to next iteration
         continue;
      if(dfs(v, visited, -1)) {    //-1 as no parent of starting vertex
         return true;
      }
   }
   return false;
}

int main() {
   bool res;
   res = hasCycle();
   if(res)
      cout << "The graph has cycle." << endl;
   else
      cout << "The graph has no cycle." << endl;
}