hdu 1143

Problem Description

In how many ways can you tile a 3xn rectangle with 2x1 dominoes? Here is a sample tiling of a 3x12 rectangle.

Input

Input consists of several test cases followed by a line containing -1. Each test case is a line containing an integer 0 ≤ n ≤ 30.

 

Output

For each test case, output one integer number giving the number of possible tilings.

 

Sample Input

2 8 12 -1

 

Sample Output

3 153 2131

 

当n为奇数时显然没有

 

a[n] = 4* a[n-2] - a[n - 4] 

 

#include <iostream>
#include <cstdio>
#include <cstring>

using namespace std;

#define N 31
long long a[N];

void init()
{
    memset(a,0,sizeof(a));
    int i;
    a[0] = 1;
    a[2] = 3;
    for(i = 3; i <= N; i++)
      if(i % 2 == 0)
        a[i] = 4*a[i-2] - a[i-4];
}

int main()
{
  init();
  int n;
  while(scanf("%d",&n))
  {
      if(n == -1)
        break;
      printf("%lld\n",a[n]);
  }
    return 0;
}