HDU 2502 月之数

原题链接

考察递推关系，a[i].cas = a[i-1].cas *2;       a[i].num = a[i-1].num * 2 - a[i-1].cas + a[i].cas;

附ac代码：

#include <stdio.h>
struct Node{
	long long cas;
	long long num;
};

int main(){
	int t, n, s, i;
	Node a[21];
	a[0].cas = a[0].num = 0;
	a[1].cas = a[1].num = 1;
	a[2].cas = 2; a[2].num = 3;
	for(i = 3; i != 21; ++i){
		a[i].cas = a[i - 1].cas * 2;
		a[i].num = a[i - 1].num * 2 - a[i- 1].cas + a[i].cas;
	}
	scanf("%d", &t);		
	while(t-- && scanf("%d", &n))
		printf("%d\n", a[n].num);	
	return 0;
}