可以参考此帖

#include <bits/stdc++.h>
using namespace std;
const int MAXN = 10000000;
int n, a[MAXN], t, ma;
bool flag;
map<int, int> m;
int main() {
    cin >> t;
    while (t--) {
        flag = ma = 0;
        m.clear();
        cin >> n;
        for (int i = 1; i <= n; i++) {
            cin >> a[i];
            m[a[i]]++;
            ma = max(ma, m[a[i]]);
        }
        if (ma >= n / 2) {
            puts("-1");
            continue;
        }
        int ans = 0;
        for (int l = 1; l <= n; l++) {
            for (int r = l + 1; r <= n; r++) {
                if (a[l] == a[r]) continue;
                for (int j = sqrt(abs(a[l] - a[r])); j >= 1; j--) {
                    if (abs(a[l] - a[r]) % j != 0) continue;
                    int cnt = 0;
                    for (int i = 1; i <= n; i++) {
                        if ((a[i] - a[l] % j) == 0) cnt++;
                        if (cnt == n / 2) {
                            ans = max(ans, j);
                            break;
                        }
                    }
                    cnt = 0;
                    int h = abs(a[l] - a[r]) / j;
                    for (int i = 1; i <= n; i++) {
                        if ((a[i] + MAXN) % h == (a[l] + MAXN) % h) cnt++;
                        if (cnt == n / 2) {
                            ans = max(ans, h);
                            break;
                        }
                    }
                }
            }
        }
        cout << ans << "\n";
    }

    return 0;
}