Mi alta precisión es bastante frustrante ...
#include <cstdio>
#include <algorithm>
#include <cstring>
using namespace std;
struct big_number{
int s[20000],cnt;
void upd(big_number &x){ //去掉前导零
while (x.s[x.cnt]==0 && x.cnt>1)
x.cnt--;
}
big_number operator + (big_number b){
big_number ret={
0};
for (int i=1,k=max(cnt,b.cnt); i<=k; i++){
ret.s[i]+=s[i]+b.s[i];
ret.s[i+1]+=ret.s[i]/10;
ret.s[i]%=10;
if (ret.s[i]<0) ret.s[i]+=10,ret.s[i+1]--;
ret.cnt=max(ret.cnt,i);
}
if (ret.s[ret.cnt+1]<0)
for (int i=1; i<=ret.cnt; i++)
ret.s[i]-=10,ret.s[i+1]++,ret.s[i+1]+=ret.s[i]/10,ret.s[i]%=10;
ret.cnt+=(ret.s[ret.cnt+1]!=0);
upd(ret);
return ret;
}
big_number operator * (big_number b){
big_number ret={
0};
for (int i=1; i<=cnt; i++){
for (int j=1; j<=b.cnt; j++){
ret.s[i+j-1]+=s[i]*b.s[j];
ret.s[i+j]+=ret.s[i+j-1]/10;
ret.s[i+j-1]%=10;
ret.cnt=max(i+j-1,ret.cnt);
}
}
ret.cnt+=(ret.s[ret.cnt+1]!=0);
upd(ret);
return ret;
}
void rd(){
char b[20000],fu=1;
scanf("%s",b); cnt=strlen(b);
if (b[0]=='-') fu=-1;
for (int i=1; i<=cnt; i++) s[i]=(b[cnt-i]-'0')*fu;
if (b[0]=='-') cnt--;
}
void pt(){
int fu=1;
if (s[cnt]<0) printf("-"),fu=-1;
for (int i=cnt; i; i--) printf("%d",s[i]*fu);
puts("");
}
} x,x1,x2,y,z,k1,k2,k3;
int main(){
int n=(scanf("%d",&n),n);
if (n==1){puts("1"); return 0;}
if (n==2){puts("5"); return 0;}
k1.s[1]=3,k1.cnt=1;
k2.s[1]=-1,k2.cnt=1;
k3.s[1]=2,k3.cnt=1;
//f[i]=(f[i-1]*3-f[i-2]+2)
z.s[1]=1;y.s[1]=5; z.cnt=y.cnt=1;
for(int i=3;i<=n;i++){
x=y*k1+z*k2+k3;
z=y,y=x;
}
y.pt();
}