Search for such a formula on OEIS, plus the multiplication method, it is AC, it is unknown. Essence Essencehttp://oeis.org/A005773

Question link:http://www.51nod.com/contest/problem.html#!problemId=1556

// Calculate .cpp 
 #include <bits/stdc ++. h> 
 #Define ll long long 
 #Define lson l, m, rt << 1 
 #Define rson m+1, r, rt << 1 | 1 
 #Define Pi 3.1415926535897932626 
 #Define exit exit (0); 
 #Define pause system ("pause"); 
 #Define Debug Puts ("Here is a bug"); 
 #Define sync_close ios :: sync_with_stdio (false); 
 #Define What_IS (X) COUT << #x << "is" << x << Endl; 
 #Define Clear (name, init) Memset (name, init, sizeof (name)); 
 const double Eps = 1E-8; 
 const int Maxn = (int) 1E6 + 5; 
 const LL MOD = 1000000007LL; 
 using namespace std; 

 Ll a [maxn]; 

 void ex_gcd (ll a, ll b, ll & x, ll & y) { 
     if (b == 0) { 
         x = 1; y = 0; 
         Return; 
     } 
     ex_gcd (b, a % b, y, x); 
     y -= x * (a/b); 
     Return; 
 } 
 int Main (int ARGC, Char Const *Argv []) {) { 
 #ifndef online_judge 
     Freopen ("D: \\ DISK_SYNCHRONOUS \\ Programs \\ ACM \\ Input.txt", "R", "R", " 
 #Endif 
     int n; cin >> n; 
     a [1] = 1; a [2] = 2; 
     for (int i = 3; i <= n; i ++) { 
         a [i] = 2*a [i-1] % mod; 
         Ll x, y; 
         ex_gcd (i, mod, x, y); 
         x = (x + mod) % mod; 
         a [i] = (a [i] + 3*(i-2)*a [i-2]%mod*x%mod)%mod; 
     } 
     COUT << A [n] << Endl; 
     Return 0; 
 }