1225. Flags

Time limit: 1.0 second
Memory limit: 64 MB

On the Day of the Flag of Russia a shop-owner decided to decorate the show-window of his shop with textile stripes of white, blue and red colors. He wants to satisfy the following conditions:

1. Stripes of the same color cannot be placed next to each other.
2. A blue stripe must always be placed between a white and a red or between a red and a white one.

Determine the number of the ways to fulfill his wish.

Example. For N = 3 result is following:

Input

N, the number of the stripes, 1 ≤ N ≤ 45.

Output

M, the number of the ways to decorate the shop-window.

Sample

3

4

1代表白色

2.蓝色

3.红色

dp[1][j]表示选到第i个条纹，第i个为j这种颜色的

然后递推

注意：

1.要用 long long

2.蓝色不能放在最后面和最前面，只能在中间

 1 #include<iostream>
 2 #include<cstring>
 3
 4 using namespace std;
 5
 6 const int maxn=48;
 7
 8 long long dp[maxn][4];
 9
10 int main()
11 {
12     dp[0][1]=dp[0][2]=dp[0][3]=0;
13     dp[1][1]=dp[1][3]=1;
14     dp[1][2]=0;
15
16     for(int i=2;i<=45;i++)
17     {
18         dp[i][1]=dp[i-1][3]+dp[i-2][3];
19         dp[i][2]=dp[i-1][1]+dp[i-1][3];
20         dp[i][3]=dp[i-1][1]+dp[i-2][1];
21     }
22
23     int n;
24
25     while(cin>>n)
26     {
27         long long  sum=dp[n][1]+dp[n][3];
28
29         cout<<sum<<endl;
30     }
31     return 0;
32 }