counts number of connected components (using dfs + t testcases)

#include <bits/stdc++.h>
using namespace std;
#define ll long long
const ll nmax=1e6+7;
bool visited[nmax]={0};
ll counter=0;
vector <ll> adj[nmax];
void clearvector(){
    for (int i=1; i<nmax; i++){
        adj[i].clear();
    }
}
void dfs(ll k){
    visited[k]=true;
    for (auto u:adj[k]){
        if (!visited[u]){
            dfs(u);
        }
    }
}
void connected(ll x){
    if (!visited[x]){
        counter++;
        dfs(x);
    }
}
int main()
{
    int t;
    cin>>t;
    while(t--){
        ll n,m;
        cin>>n>>m;
        for (ll i=0; i<m; i++){
            ll x,y;
            cin>>x>>y;
            adj[x].push_back(y);
            adj[y].push_back(x);
        }
        for (ll i=1; i<=n; i++){
            connected(i);
        }
        cout<<counter<<"\n";
        clearvector();
        visited[nmax]={0};
    }
}