一个数n，表示长度。(n<1e15)

长度为n的非010串的个数。(对1e9+7取模)

3

7解释:
000
001
011
100
101
110
111

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;const long long mod=1000000007;struct matrix
{long long a[3][3];
};matrix cal(matrix A,matrix B)
{int i,j,k;matrix C;for(i=0;i<3;i++){for(j=0;j<3;j++){C.a[i][j]=0;for(k=0;k<3;k++){C.a[i][j]=(C.a[i][j]+(A.a[i][k]*B.a[k][j])%mod+mod)%mod;}}}return C;
}int out(matrix A,matrix B)
{cout<<"s:"<<endl;cout<<A.a[0][0]<<" "<<A.a[0][1]<<" "<<A.a[0][2]<<endl;cout<<A.a[1][0]<<" "<<A.a[1][1]<<" "<<A.a[1][2]<<endl;cout<<A.a[2][0]<<" "<<A.a[2][1]<<" "<<A.a[2][2]<<endl;cout<<"base:"<<endl;cout<<B.a[0][0]<<" "<<B.a[0][1]<<" "<<B.a[0][2]<<endl;cout<<B.a[1][0]<<" "<<B.a[1][1]<<" "<<B.a[1][2]<<endl;cout<<B.a[2][0]<<" "<<B.a[2][1]<<" "<<B.a[2][2]<<endl;return 0;
}matrix pow_mod(long long x)
{matrix s,base;base.a[0][0]=2;base.a[1][0]=-1;base.a[2][0]=1;base.a[0][1]=1;base.a[1][1]=base.a[2][1]=0;base.a[1][2]=1;base.a[0][2]=base.a[2][2]=0;s.a[0][0]=7;s.a[0][1]=4;s.a[0][2]=2;s.a[1][0]=s.a[1][1]=s.a[1][2]=0;s.a[2][0]=s.a[2][1]=s.a[2][2]=0;out(s,base);while(x){if(x&1)s=cal(s,base);base=cal(base,base);out(s,base);x>>=1;}return s;
}int main()
{long long n;long long ans;while(scanf("%lld",&n)!=EOF){if(n==0)cout<<0<<endl;else if(n==1)cout<<2<<endl;else if(n==2)cout<<4<<endl;else if(n==3)cout<<7<<endl;else{matrix t=pow_mod(n-3);ans=t.a[0][0]%mod;cout<<ans<<endl;/*cout<<t.a[0][1]%mod<<endl;cout<<t.a[1][0]%mod<<endl;cout<<t.a[1][1]%mod<<endl;*/}}return 0;
}