传送门

文章目录

题意：

思路：

显然是多重集排列数，我们考虑构造指数生成函数，让后模拟一下多项式乘法即可啦。

由于存在分数，所以直接用doubledoubledouble即可。

//#pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math")//#pragma GCC target("sse,sse2,sse3,ssse3,sse4.1,sse4.2,avx,avx2,popcnt,tune=native")//#pragma GCC optimize(2)#include<cstdio>#include<iostream>#include<string>#include<cstring>#include<map>#include<cmath>#include<cctype>#include<vector>#include<set>#include<queue>#include<algorithm>#include<sstream>#include<ctime>#include<cstdlib>#include<random>#include<cassert>#define X first#define Y second#define L (u<<1)#define R (u<<1|1)#define pb push_back#define mk make_pair#define Mid ((tr[u].l+tr[u].r)>>1)#define Len(u) (tr[u].r-tr[u].l+1)#define random(a,b) ((a)+rand()%((b)-(a)+1))#define db puts("---")using namespace std;//void rd_cre() { freopen("d://dp//data.txt","w",stdout); srand(time(NULL)); }//void rd_ac() { freopen("d://dp//data.txt","r",stdin); freopen("d://dp//AC.txt","w",stdout); }//void rd_wa() { freopen("d://dp//data.txt","r",stdin); freopen("d://dp//WA.txt","w",stdout); }typedef long long LL;typedef unsigned long long ULL;typedef pair<int,int> PII;const int N=1000010,mod=1e9+7,INF=0x3f3f3f3f;const double eps=1e-6;int n,m;int a[N];double x[N],y[N];int fun(int x) {int ans=1;for(int i=1;i<=x;i++) ans*=i;return ans;}int main(){//ios::sync_with_stdio(false);//cin.tie(0);while(cin>>n>>m) {for(int i=1;i<=n;i++) cin>>a[i];for(int j=0;j<=m;j++) x[j]=y[j]=0;for(int i=0;i<=a[1];i++) x[i]=1.0/fun(i);for(int i=2;i<=n;i++) {for(int j=0;j<=m;j++) {for(int k=0;k+j<=m&&k<=a[i];k++) {(y[j+k]+=x[j]/fun(k));}}for(int j=0;j<=m;j++) x[j]=y[j],y[j]=0;}printf("%.0f\n",(x[m]*fun(m)));}return 0;}/**/