题解 | 畅通工程

#include <iostream>
#include <vector>
#include <cstring>
using namespace std;

void dfs(int j, vector<vector<int>>& a, vector<int>& v) {
	v[j] = 1;
	for (int i = 0; i < a[j].size(); i++) {
		if (!v[a[j][i]])dfs(a[j][i], a, v);
	}
}
int main() {
	int N, M;
	while (cin >> N >> M) {
		if (N == 0)break;
		vector<int> v(N + 1, 0);
		vector<vector<int>> a(N + 1);
		int x, y;
		for (int i = 0; i < M; i++) {
			cin >> x >> y;
			a[x].push_back(y);
			a[y].push_back(x);
		}
		int t = 0;
		for (int i = 1; i <= N; i++) {
			if (!v[i]) {
				dfs(i, a, v);
				t++;
			}
		}
    	cout << t - 1 << endl;
	}
}