暴力枚舉前幾項後發現規律
import java.math.*;
import java.util.Scanner;
public class Main{
public static void main(String[] args){
Scanner cin = new Scanner(System.in);
BigInteger[] a = new BigInteger[1010];
a[1].valueOf(0);
a[2].valueOf(1);
for (int i = 3; i<1010;++i){
a[i] = a[i].add(a[i-1]);
a[i] = a[i].add(a[i-2]);
a[i] = a[i].add(a[i-2]);
}
while(cin.hasNext()) {
int n = cin.nextInt();
System.out.println(a[n]);
}
}
}