1556 计算

 1 #include<stdio.h>
 2 #include<algorithm>
 3 #include<iostream>
 4 #include<string.h>
 5 #include<queue>
 6 #include<math.h>
 7 #include<stack>
 8 #include<vector>
 9 using namespace std;
10 typedef long long LL;
11 const LL mod = 1e9+7;
12 LL M[1000005];
13 LL quick(LL n,LL m);
14 int main(void)
15 {
16         M[1] = 1;
17         M[2] = 2;M[0] = 1;
18         int n;
19         scanf("%d",&n);
20          for(int i = 3; i <= n; i++)
21         {
22                 LL ni = i+2;
23                 ni = quick(ni,mod-2);
24                 M[i] = (LL)(2*i+1)*M[i-1]%mod+(LL)3*(i-1)*M[i-2]%mod;
25                 M[i] %= mod;
26                 M[i] = M[i]*ni%mod;
27         }
28         LL sum = quick((LL)3,(LL)n-1);
29         for(int i = 2;i <= n; i++)
30         {
31             LL ak = M[i-2]*quick((LL)3,(LL)n-i)%mod;
32             sum = sum - ak;
33             sum%=mod;
34         }
35         printf("%lld\n",(sum%mod+mod)%mod);
36         return 0;
37 }
38 LL quick(LL n,LL m)
39 {
40         LL ask = 1;
41         while(m)
42         {
43                 if(m&1)
44                         ask = ask*n%mod;
45                 n = n*n%mod;
46                 m/=2;
47         }
48         return ask;
49 }