1272. Non-Yekaterinburg Subway - Timus

#include <bits/stdc++.h>

using namespace std;

vector < vector < int > > G;
vector < bool > viz;

void DFS(int node) {
    viz[node] = true;
    for(int next : G[node])
        if(!viz[next])
            DFS(next);
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    int N, K, M;
    cin >> N >> K >> M;
    G.resize(N + 1);
    viz.resize(N + 1);
    while(K--) {
        int u, v;
        cin >> u >> v;
        G[u].emplace_back(v);
        G[v].emplace_back(u);
    }
    int cnt = 0;
    for(int i = 1; i <= N; ++i)
        if(!viz[i]) {
            ++cnt;
            DFS(i);
        }
    cout << cnt - 1;
}