在计算中找规律（大胆猜想 + 逆向思维）

#include 
#include 
using namespace std;
typedef long long LL;
const LL N = 1e6 + 10;
LL n;
LL a[N]; 
LL ans = 0;

int main()
{
	ios::sync_with_stdio(false);
	cin.tie(0), cout.tie(0);
	cin >> n;
	for (int i = 1; i <= n; ++i)
	{
		cin >> a[i];
	}
	if (n == 1)
		cout << a[1] << ".0000000000" << endl;
	else if (n == 2)
		cout << abs(a[2] - a[1]) << ".0000000000" << endl;	
	else
	{
		LL temp = abs(a[n] - a[n - 1]);
		for (int i = n - 2; i >= 1; --i)
		{
			ans = abs(a[i] - temp);
			temp = ans;
		}
		cout << ans << ".0000000000" << endl;
	}	
	return 0;
}