1: 1
2:1+(1+1)
3:1+2+(2+3)
4:1+2+5+(5+8)
而斐波那契數列1 1 2 3 5 8……
因此推出a[n]=a[n-1]+fib[2*i-1]+fib[2*1-2];
java代碼
import java.util.*;
import java.math.*;
public class Main {
public static void main(String args[]){
BigInteger [] ans=new BigInteger[2010];
ans[1]= new BigInteger("1");
ans[2]= new BigInteger("3");
BigInteger [] fib=new BigInteger[5020];
fib[1]=new BigInteger("1");
fib[2]=new BigInteger("1");
for(int i=3;i<5020;i++){
fib[i]=fib[i-1].add(fib[i-2]);
}
for(int i=3;i<2010;i++){
ans[i]=ans[i-1].add(fib[2*i-3].add(fib[2*i-2]));
}
Scanner read=new Scanner(System.in);
int n;
n=read.nextInt();
while(n!=0){
System.out.println(ans[n]);
n=read.nextInt();
}
}
}
當然也有其他的方法,比如stainger的解法,c++代碼
#include <iostream>
#include <cstdio>
#include <cstring>
#include <string>
#include <algorithm>
using namespace std;
struct Bigint
{
string a;
int sign;
Bigint() {}
Bigint( string b ) { (*this) = b; }
int size() {
return a.size();
}
Bigint inverseSign() {
sign *= -1;
return (*this);
}
Bigint normalize( int newSign ) {
for( int i = a.size() - 1; i > 0 && a[i] == '0'; i-- )
a.erase(a.begin() + i);
sign = ( a.size() == 1 && a[0] == '0' ) ? 1 : newSign;
return (*this);
}
void operator = ( string b ) {
a = b[0] == '-' ? b.substr(1) : b;
reverse( a.begin(), a.end() );
this->normalize( b[0] == '-' ? -1 : 1 );
}
bool operator < ( const Bigint &b ) const {
if( sign != b.sign ) return sign < b.sign;
if( a.size() != b.a.size() )
return sign == 1 ? a.size() < b.a.size() : a.size() > b.a.size();
for( int i = a.size() - 1; i >= 0; i-- ) if( a[i] != b.a[i] )
return sign == 1 ? a[i] < b.a[i] : a[i] > b.a[i];
return false;
}
bool operator == ( const Bigint &b ) const {
return a == b.a && sign == b.sign;
}
Bigint operator + ( Bigint b ) {
if( sign != b.sign ) return (*this) - b.inverseSign();
Bigint c;
for(int i = 0, carry = 0; i<a.size() || i<b.size() || carry; i++ ) {
carry+=(i<a.size() ? a[i]-48 : 0)+(i<b.a.size() ? b.a[i]-48 : 0);
c.a += (carry % 10 + 48);
carry /= 10;
}
return c.normalize(sign);
}
Bigint operator - ( Bigint b ) {
if( sign != b.sign ) return (*this) + b.inverseSign();
int s = sign; sign = b.sign = 1;
if( (*this) < b ) return ((b - (*this)).inverseSign()).normalize(-s);
Bigint c;
for( int i = 0, borrow = 0; i < a.size(); i++ ) {
borrow = a[i] - borrow - (i < b.size() ? b.a[i] : 48);
c.a += borrow >= 0 ? borrow + 48 : borrow + 58;
borrow = borrow >= 0 ? 0 : 1;
}
return c.normalize(s);
}
Bigint operator * ( Bigint b ) {
Bigint c("0");
for( int i = 0, k = a[i] - 48; i < a.size(); i++, k = a[i] - 48 ) {
while(k--) c = c + b;
b.a.insert(b.a.begin(), '0');
}
return c.normalize(sign * b.sign);
}
Bigint operator / ( Bigint b ) {
if( b.size() == 1 && b.a[0] == '0' ) b.a[0] /= ( b.a[0] - 48 );
Bigint c("0"), d;
for( int j = 0; j < a.size(); j++ ) d.a += "0";
int dSign = sign * b.sign; b.sign = 1;
for( int i = a.size() - 1; i >= 0; i-- ) {
c.a.insert( c.a.begin(), '0');
c = c + a.substr( i, 1 );
while( !( c < b ) ) c = c - b, d.a[i]++;
}
return d.normalize(dSign);
}
Bigint operator % ( Bigint b ) {
if( b.size() == 1 && b.a[0] == '0' ) b.a[0] /= ( b.a[0] - 48 );
Bigint c("0");
b.sign = 1;
for( int i = a.size() - 1; i >= 0; i-- ) {
c.a.insert( c.a.begin(), '0');
c = c + a.substr( i, 1 );
while( !( c < b ) ) c = c - b;
}
return c.normalize(sign);
}
void print() {
if( sign == -1 ) putchar('-');
for( int i = a.size() - 1; i >= 0; i-- ) putchar(a[i]);
}
};
int main()
{
Bigint ans[2010];
ans[1]="1",ans[2]="3";
for(int i=3;i<2005;i++){
ans[i]=ans[i-1]+ans[i-1]+ans[i-1]-ans[i-2];
}
int n;
while(~scanf("%d",&n)&&n!=0){
ans[n].print();
cout<<endl;
}
return 0;
}