递推DP URAL 1225 Flags

题目传送门

 /*
     1 r; 2 b; 3 w
     2不能在最前面，所以dp[1] = 2;    dp[2] = 2: 13 or 31

     dp[i] = dp[i-1] + dp[i-2];
     只加1或3时，总数dp[i-1];    只加12或32时，总数dp[i-2];
     详细解释：http://www.cnblogs.com/vongang/archive/2011/09/30/2196847.html
 */
 #include <cstdio>
 #include <iostream>
 #include <algorithm>
 #include <cstring>
 using namespace std;

 const int MAXN = ;
 const int INF = 0x3f3f3f3f;
 long long dp[MAXN];

 int main(void)        //URAL 1225 Flags
 {
     //freopen ("A.in", "r", stdin);

     int n;
     while (scanf ("%d", &n) == )
     {
         memset (dp, , sizeof (dp));

         dp[] = ;    dp[] = ;
         for (int i=; i<=n; ++i)
         {
             dp[i] = dp[i-] + dp[i-];
         }

         printf ("%I64d\n", dp[n]);
     }

     return ;
 }