HDU 2045 不容易系列之(3)—— LELE的RPG难题

#include <iostream>
#include <cstring>
#include <cstdio>
#include <cmath>
#include <set>
#include <stack>
#include <cctype>
#include <algorithm>
#define lson o<<1, l, m
#define rson o<<1|1, m+1, r
using namespace std;
typedef long long LL;
const int maxn = 500050;
const int mod = 99999997;
const int MAX = 0x3f3f3f3f;
int t, a, b;
LL f[100];
int main()
{
    f[1] = 3;
    f[2] = 6;
    f[3] = 6;
    for(int i = 4; i <= 50; i++) f[i] = f[i-1] + 2*f[i-2];
    while(~scanf("%d", &b)) {
        printf("%I64d\n", f[b]);
    }
    return 0;
}