For which input output is coming wrong..confusion!

I am confused for which input the output is coming wrong because it is not accepted on spoj as wrong answer is being shown.
My code is shown below–

#include<bits/stdc++.h>
using namespace std;
class Graph
{
int v;
list *adj;
public:
Graph(int v);
void addEdge(int v,int w);
void DFS(int s);
};
Graph::Graph(int v)
{
this->v=v;
adj=new list[v];
}
void Graph:: addEdge(int v,int w)
{
adj[v].push_back(w);
adj[w].push_back(v);
}
void Graph:: DFS(int s)
{
int fvar=s;
int flag=1;
vector visited(v,false);
stack stk;
vectorparent(v,-1);
stk.push(s);
while(!stk.empty() && flag!=0){
s=stk.top();
stk.pop();
if(!visited[s]){
visited[s]=true;
}
list:: iterator it;
for(it=adj[s].begin();it!=adj[s].end();it++){

        if(!visited[*it]){
            parent[*it]=s;
            stk.push(*it);
        }
    if(visited[*it] && parent[s]!=*it){
        flag=0;
        break;
    }
    }
}
vector<bool>:: iterator it1;
for(it1=visited.begin();it1!=visited.end();it1++){
    if(visited[*it1]==false){
        flag=0;
        break;
    }
}
if(flag==0)
    cout<<"NO"<<endl;
else
    cout<<"YES"<<endl;

}
int main()
{
int i,m,x,y,v;
cin>>v>>m;
Graph g(v);
for(i=1;i<=m;i++){
cin>>x>>y;
g.addEdge(x-1,y-1);
}

g.DFS(x-1);
return 0;

}

Plz help. I have made my program by considering the vertices starting from zero thats why i have used x-1 and y-1 in addEdge function. Hope you understand.