[bzoj1002]轮状病毒

#include<cstdio>
#include<cstring>
#define inf 0x3f3f3f3f
#define r register
#define min(a,b) (a<b?a:b)
struct AriM{
    int d[101],len;
    AriM friend operator *(AriM a,int k){
        for(r int i=1;i<=a.len;i++)a.d[i]*=k;
        for(r int i=1;i<=a.len;i++){
            a.d[i+1]+=a.d[i]/10;
            a.d[i]%=10;
        }
        if(a.d[a.len+1])a.len++;
        return a;
    }
    AriM friend operator -(AriM a,AriM b){
        a.d[1]+=2;
        for(r int j=1;a.d[j]>=10;a.d[++j]++)a.d[j]%=10;
        for(r int i=1;i<=a.len;i++){
               a.d[i]-=b.d[i];
               if(a.d[i]<0)a.d[i]+=10,a.d[i+1]--;
        }
        while(!a.d[a.len])a.len--;
        return a;
    }
}f[101]; 
int main(){
    int n;
    scanf("%d",&n);
    f[1].d[1]=1;f[1].len=1;
    f[2].d[1]=5;f[2].len=1;
    for(r int i=3;i<=n;i++)f[i]=f[i-1]*3-f[i-2];
    for(r int i=f[n].len;i;i--)printf("%d",f[n].d[i]);
    puts("");
    return 0;
}