用容器vector即可，vector之前在我的题解中讲过，所以不讲了

$AC$ $Code$

#include <bits/stdc++.h>
using namespace std;
int n, m, sum;
vector<int> v[100005];
int main()
{
    cin >> n >> m;
    for (int i = 1; i <= m; i++)
    {
        int x, y;
        cin >> x >> y;
        v[x].push_back(y);
    }
    for (int i = 1; i <= n; i++)
    {
        sort(v[i].begin(), v[i].end());
        cout << v[i].size() << " ";
        for (int j = 0; j < v[i].size(); j++)
            cout << v[i][j] << " ";
        cout << endl;
    }
    return 0;
}