#include <iostream>
#include <algorithm>
#include <unordered_map>
using namespace std;
const int N = 1e6 + 5;
unordered_map<int, int> check;
struct node
{
    int v, id;
} e[N];
bool cmp(node a, node b)
{
    if (a.v == b.v)
        return a.id < b.id;
    return a.v < b.v;
}
int main()
{
    ios::sync_with_stdio(false), cin.tie(0);
    int n, m, tot, top;
    long long ans = 9e18;
    cin >> n >> m;
    for (int i = 1; i <= m; i++)
    {
        cin >> tot;
        for (int j = 1; j <= tot; j++)
            top++, cin >> e[top].v, e[top].id = i;
    }
    sort(e + 1, e + 1 + n, cmp);
    int i = 1, j = 0, cnt = 0, p1 = 1, p2 = n;
    while (i <= n)
    {
        while (cnt < m && j < n)
        {
            j++;
            if (!check[e[j].id])
                cnt++;
            check[e[j].id]++;
        }
        if (ans > e[j].v - e[i].v && cnt == m)
            ans = e[j].v - e[i].v, p1 = i, p2 = j;
        check[e[i].id]--;
        if (!check[e[i].id])
            cnt--;
        i++;
    }
    cout << e[p2].v - e[p1].v << endl;
}

双指针写法，在别处已经AC了，在这里全RE，并且在我的VScode里也是RE，什么原因呢？