hdu 2045 RPG难题

#include <stdio.h> int main() { int n,i; __int64 a[51]; a[1] = 3; a[2] = 6; a[3] = 6; for(i = 4;i <= 50;i ++) { a[i] = a[i-1] + a[i-2]*2; } while(scanf("%d",&n)!=EOF) { printf("%I64d\n",a[n]); } return 0; }