B. Permutation Swap

#include <bits/stdc++.h>

using namespace std;

int main() {
    ios_base::sync_with_stdio(false), cin.tie(NULL), cout.tie(NULL);

    int testcase;
    cin >> testcase;
    while(testcase--) {
        int n;
        cin >> n;

        set <int> s;
        for (int i = 1; i <= n; i++) {
            int x;
            cin >> x;
            if (abs(x - i) != 0) s.insert(abs(x - i));
        }
        int ans = 0;

        for (auto i : s) {
            ans = __gcd(ans, i);
        }

        cout << ans << '\n';
    }

    return 0;
}