#ifdef DEBUG
#define _GLIBCXX_DEBUG
#endif
//#pragma GCC optimize("O3")
#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
int k;
void solve() {
    cin >> k;
    vector<vector<ll>> a(k);
    vector<pair<ll,int>> t;
    for (int i = 0; i < k; i++) {
        int sz;
        cin >> sz;
        a[i].resize(sz);
        for (ll& u : a[i]) {
            cin >> u;
            t.emplace_back(u, i);
        }
    }
    sort(t.begin(), t.end());
    vector<ll> mn(k);
    multiset<ll> vals;
    for (int i = 0; i < k; i++) {
        mn[i] = 1e18;
        vals.insert(mn[i]);
    }
    ll best = 1e18;
    reverse(t.begin(), t.end());
    for (auto& it : t) {
        int id = it.second;
        vals.erase(vals.find(mn[id]));
        mn[id] = it.first;
        vals.insert(mn[id]);
        best = min(best, *(--vals.end()) - mn[id]);
    }
    cout << best << '\n';
}
int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
#ifdef DEBUG
    freopen("input.txt", "r", stdin);
#endif
    int tst;
    cin >> tst;
    while (tst--) {
        solve();
    }
    return 0;
}