1828B - Permutation Swap - CodeForces Solution

   #include<bits/stdc++.h>
   using namespace std;

   #define ll long long

   int main(){
   	 int t;
   	 cin>>t;
   	 while(t--){
   	 	int n;
   	 	cin >> n;
   	 	vector<int> arr(n);
   	 	vector<int> v;
   	 	for(int i=0;i < n;i++){
   	 		cin >> arr[i];
   	 		if(arr[i] != (i+1)){
   	 			v.push_back(abs(arr[i] - (i+1))) ;
   	 		}
   	 	}

   	 	int ans = v[0];
   	 	for(int i=1;i<v.size();i++){
   	 		ans = __gcd(ans,v[i]);
   	 	}

   	 	cout << ans << endl;

   	 }

   	 return 0;
   }