cycle detection in undirected graph

 /*
 	 ->pass currvertex and it's parent as -1 as argument to the dfs function
     ->traverse for thr adjancy list of the curr vertex
     		->if child is not visited --> call dfs ans pass child and it's parent as curr vertex
            ->if child is visisited
            	->if that child is not parent --> return true
                
       
       ->return false
       
       Here,basically we are checking the back edge
 
 
 */
			vector g[100001];
            vector visi(100001, 0);

           // -->cycle detection for undirected graph
           bool check_cycle(ll v, ll p)
          {
              visi[v] = true;
          
          
              for (ll child : g[v]) {
                  if (visi[child] == false) {
                      if(check_cycle(child, v)==true)
                          return true;
                  }
                  else {
                      if (child != p)
                          return true;
                  }
              }
              return false;
          }

          //here,p passed in a dfs function is a parent of a 'v' node,at start we pass dfs(v,-1);