BZOJ 1002: [FJOI2007]轮状病毒 []

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cmath>
using namespace std;
const int N=100;
int n;
struct Big{
    int d[N],l;
    int& operator [](int x){return d[x];}
    Big(){l=1;memset(d,0,sizeof(d));}
}f[105];
Big operator *(Big a,int b){
    int g=0;
    for(int i=1;i<=a.l;i++){
        g+=a[i]*b;
        a[i]=g%10;
        g/=10;
    }
    for(;g;g/=10) a[++a.l]=g%10;
    return a;
}    
Big operator -(Big a,Big b){
    for(int i=1;i<=b.l;i++){
        if(a[i]<b[i]) a[i]+=10,a[i+1]--;
        a[i]-=b[i];
    }
    int p=b.l+1;
    while(a[p]<0) a[p]+=10,a[p+1]--;
    while(a[a.l]==0) a.l--;
    return a;
}
Big operator +(Big a,int b){
    int g=b,i=1;
    for(;g;i++) g+=a[i],a[i]=g%10,g/=10;
    a.l=max(a.l,i);
    return a;
}
void print(Big &a){
    for(int i=a.l;i>=1;i--) printf("%d",a[i]);
}    
int main(){
    scanf("%d",&n);
    f[1].l=1;f[1][1]=1;
    f[2].l=1;f[2][1]=5;
    for(int i=3;i<=n;i++) f[i]=f[i-1]*3-f[i-2]+2;
    print(f[n]);
}