ural 1225. Flags

纠结啊。。。。贴道水题。

 

我推的公式，用排列组合，过后看下discuss，居然是斐波那契数列的二倍。。情何以堪！！！

 

#include <queue>#include <stack>#include <math.h>#include <stdio.h>#include <stdlib.h>#include <iostream>#include <limits.h>#include <string.h>#include <string>#include <algorithm>using namespace std;const int MAX = 50;long long dp[MAX];long long c[MAX][MAX];int main(){int n;fill(dp,dp+MAX,2);memset(c,0,sizeof(c));for(int i=0; i<MAX; i++){c[i][0] = 1;c[i][1] = i;}for(int i=1; i<MAX; i++)for(int k=1; k<=i; k++)c[i][k] = (c[i-1][k] + c[i-1][k-1]);for(int i=3; i<=45; i++)for(int k=i-2; k>=0; k-- ){if( i-k-1 > k )break;dp[i] += 2*c[k][i-k-1];}scanf("%d",&n);printf("%lld/n",dp[n]);return 0;}