Connectivity

#include <iostream>
#include <cmath>
#include <cstring>
#include <algorithm>
#include <vector>

using namespace std;

int n, m, cnt = 0, a, b, v, arr[101][101], lamps[101];

void dfs(int v)
{
    cnt++;
    lamps[v] = 1;
//  cout << v << " ";

    for (int i = 0; i < n; i++)
    {
        if(arr[v][i] == 1 && lamps[i] == 0)
        {
            dfs(i);
        }
    }
}

int main()
{
    //--------------------------------
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    //--------------------------------

    cin >> n >> m;

    for (int i = 0; i < m; i++)
    {
        cin >> a >> b;
        arr[a-1][b-1] = arr[b-1][a-1] = 1;
    }

    dfs(0);

    (cnt == n) ? (cout << "YES") : (cout << "NO");

    return 0;
}