递推DP URAL 1225 Flags

 

题目传送门

 1 /*  2  1 r; 2 b; 3 w  3  2不能在最前面，所以dp[1] = 2; dp[2] = 2: 13 or 31  4  5  dp[i] = dp[i-1] + dp[i-2];  6  只加1或3时，总数dp[i-1]; 只加12或32时，总数dp[i-2];  7  详细解释：http://www.cnblogs.com/vongang/archive/2011/09/30/2196847.html  8 */  9 #include <cstdio> 10 #include <iostream> 11 #include <algorithm> 12 #include <cstring> 13 using namespace std; 14 15 const int MAXN = 55; 16 const int INF = 0x3f3f3f3f; 17 long long dp[MAXN]; 18 19 int main(void) //URAL 1225 Flags 20 { 21 //freopen ("A.in", "r", stdin); 22 23 int n; 24 while (scanf ("%d", &n) == 1) 25  { 26 memset (dp, 0, sizeof (dp)); 27 28 dp[1] = 2; dp[2] = 2; 29 for (int i=3; i<=n; ++i) 30  { 31 dp[i] = dp[i-1] + dp[i-2]; 32  } 33 34 printf ("%I64d\n", dp[n]); 35  } 36 37 return 0; 38 }