这个题是个递推题:
f0=1;
f1=3;
fi=3*f(i-1)-f(i-2);
使用大数,大数以1e16为一进制,最后注意输出格式,wa了4发
#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
typedef long long LL;
const int maxn=2000;
const int n2=maxn/16;
const LL base=1e16;
LL fib[maxn+2][n2+2];
void setfib()
{
fib[1][0]=1;
fib[2][0]=3;
int len=1;
for(int i=3;i<=maxn;i++){
int carry=0;
for(int j=0;j<len;j++){
fib[i][j]=3*fib[i-1][j]-fib[i-2][j]+carry;
if(fib[i][j]<0){
carry=-1;
fib[i][j]+=base;
}else{
carry=fib[i][j]/base;
fib[i][j]%=base;
}
}
if(carry){
fib[i][len++]=carry;
}
}
}
int main()
{
memset(fib,0,sizeof(fib));
setfib();
int n;
while(scanf("%d",&n)!=EOF&&n){
int k=n2;
while(fib[n][k]==0){
k--;
}
printf("%lld",fib[n][k]);
for(int i=k-1;i>=0;i--){
printf("%016lld",fib[n][i]);//格式
}
printf("\n");
}
return 0;
}