[hdoj]1143递归水题……可是我还没做出来……

 1 #include <stdio.h>  
 2 #define MAX 31  
 3 int a[MAX];  
 4 int main() {
 5     int n;
 6     a[0] = 1;  
 7     a[2] = 3;  
 8     for(int i = 4; i< MAX; i = i+2)  
 9         a[i] = 4*a[i-2] - a[i-4];  
10     while(scanf("%d",&n)==1&&n!=-1)         
11         printf("%d\n",a[n]);
12     return 0;  
13 }