908D New Year and Arbitrary Arrangement
分析
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代码
#include<iostream> #include<cstdio> #include<cstring> #include<string> #include<algorithm> #include<cctype> #include<cmath> #include<cstdlib> #include<queue> #include<ctime> #include<vector> #include<set> #include<map> #include<stack>
using namespace std; const int mod = 1e9+7; int k,x,y,dp[2010][2010]; inline int pw(int a,int p){ int res=1; while(p){ if(p&1)res=1ll*res*a%mod; a=1ll*a*a%mod; p>>=1; } return res; } inline int go(int i,int j){ if(i+j>=k)return dp[i][j]=(i+j+1ll*x*pw(y,mod-2)%mod)%mod; if(dp[i][j]!=-1)return dp[i][j]; dp[i][j]=1ll*(1ll*go(i+1,j)*x%mod+1ll*go(i,j+i)*y%mod)*pw(x+y,mod-2)%mod; return dp[i][j]; } int main(){ cin>>k>>x>>y; memset(dp,-1,sizeof(dp)); cout<<go(1,0); return 0; }
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