代码：

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

struct node {
	int v;
	int next;
} a[100005 * 2], adj[5002];
int n, m, fa = 0;

void insert(int x, int y) {
	a[++fa].v = y;
	a[fa].next = adj[x].next;
	adj[x].next = fa;
	adj[x].v++;
}

int main() {
	cin >> n >> m;
	for (int i = 0; i < m; i++) {
		int u, v;
		cin >> u >> v;
		insert(u, v);
		insert(v, u);
	}
	for (int i = 1; i <= n; i++) {
		cout << adj[i].v << " ";
		for (int hd = adj[i].next; hd; hd = a[hd].next) {
			cout << a[hd].v << " ";
		}
		cout << "\n";
	}
	return 0;
}

题目:T465887