Cycle Detection in connected undirected graph using BFS (Implementation using STL Cpp)

Program:-

#include <bits/stdc++.h>
using namespace std;

int main() {
    int n,m;
    cin>>n>>m;
    vector<int>vec[n];
    for(int i=0;i<m;i++){
        int a,b;
        cin>>a>>b;
        a--;b--;
        vec[a].push_back(b);
        vec[b].push_back(a);
    }
    vector<int>dist(n,-1);
    dist[0]=0;//starting number is always 0..
    queue<int>que;
    que.push(0);
    while(!que.empty()){
        int index=que.front();que.pop();
        for(int i=0;i<vec[index].size();i++){
            if(dist[vec[index][i]]==-1){
                dist[vec[index][i]]=dist[index]+1;
                que.push(vec[index][i]);
            }
          
        }
    }
    if(n==1) cout<<"Not Cycle";
  
    for(int i=1;i<n;i++){
         set<int>st;
        for(int j=0;j<vec[i].size();j++)
            st.insert(dist[vec[i][j]]);
            st.insert(dist[i]);
        if(vec[i].size()>1 && st.size()<=vec[i].size()){ cout<<"Cycle";return 0;}
    }
    cout<<"Not Cycle";
   
    return 0;
}