CF problem 166E Tetrahedron

Find the law, f [n] and f [n-1] and f [N-2] Obviously related...

 

?#include<iostream>  #include<algorithm>  #include<vector>  #include<string>  #include<stack>  #include<queue>  #include<map>  #include<set>  #include<cstring>  #include<cstdio>  #include<cstdlib>   using namespace std;  #define FF(i, a, b) for(i=a; i<b; i++)  #define FD(i, a, b) for(i=a; i>b; i--)  #define CLR(a, b) memset(a, b, sizeof(b))  #define CPY(a, b) memcpy(a, b, sizeof(a))   long long n; long long f[10000001];  int main() {     f[1] = 0;     f[2] = 3;     f[3] = 6;     int i;     FF(i, 4, 10000001)     {         f[i] = f[i-1] * 2 + f[i-2] * 3;         f[i] %= 1000000007;     }     while(cin>>n) cout<<f[n]<<endl;     return 0; } #include<iostream>#include<algorithm>#include<vector>#include<string>#include<stack>#include<queue>#include<map>#include<set>#include<cstring>#include<cstdio>#include<cstdlib>using namespace std;#define FF(i, a, b) for(i=a; i<b; i++)#define FD(i, a, b) for(i=a; i>b; i--)#define CLR(a, b) memset(a, b, sizeof(b))#define CPY(a, b) memcpy(a, b, sizeof(a))long long n;long long f[10000001];int main(){    f[1] = 0;    f[2] = 3;    f[3] = 6;    int i;    FF(i, 4, 10000001)    {        f[i] = f[i-1] * 2 + f[i-2] * 3;        f[i] %= 1000000007;    }    while(cin>>n) cout<<f[n]<<endl;    return 0;}