题意：给定n件物品，每件物品让周驿东开心的概率为a[i]
要求从中选一些，使得周驿东恰好开心一次的概率最大
n<=1e4，0<=a[i]<=1
思路：
1 #include<bits/stdc++.h> 2 using namespace std; 3 typedef long long ll; 4 typedef unsigned int uint; 5 typedef unsigned long long ull; 6 typedef pair<int,int> PII; 7 typedef pair<ll,ll> Pll; 8 typedef vector<int> VI; 9 typedef vector<PII> VII; 10 #define N 21000 11 #define M 4100000 12 #define fi first 13 #define se second 14 #define MP make_pair 15 #define pi acos(-1) 16 #define mem(a,b) memset(a,b,sizeof(a)) 17 #define rep(i,a,b) for(int i=(int)a;i<=(int)b;i++) 18 #define per(i,a,b) for(int i=(int)a;i>=(int)b;i--) 19 #define lowbit(x) x&(-x) 20 #define Rand (rand()*(1<<16)+rand()) 21 #define id(x) ((x)<=B?(x):m-n/(x)+1) 22 #define ls p<<1 23 #define rs p<<1|1 24 25 const ll MOD=1e9+7,inv2=(MOD+1)/2; 26 double eps=1e-6; 27 int INF=1e9; 28 int dx[4]={-1,1,0,0}; 29 int dy[4]={0,0,-1,1}; 30 31 double a[N],b[N],c[N],t1[N],t2[N]; 32 33 int read() 34 { 35 int v=0,f=1; 36 char c=getchar(); 37 while(c<48||57<c) {if(c=='-') f=-1; c=getchar();} 38 while(48<=c&&c<=57) v=(v<<3)+v+v+c-48,c=getchar(); 39 return v*f; 40 } 41 42 int main() 43 { 44 //freopen("1.in","r",stdin); 45 //freopen("1.out","w",stdout); 46 47 int cas; 48 scanf("%d",&cas); 49 50 while(cas--) 51 { 52 int n=read(); 53 double ans=0; 54 int m=0; 55 rep(i,1,n) 56 { 57 scanf("%lf",&a[i]); 58 if(a[i]<=0.5) b[++m]=a[i]; 59 ans=max(ans,a[i]); 60 } 61 sort(b+1,b+1+m); 62 double p=0,w=1.0; 63 per(i,m,1) 64 { 65 double np=p*(1.0-b[i])+w*b[i]; 66 double nw=w*(1.0-b[i]); 67 if(np<=p) break; 68 p=np; 69 w=nw; 70 } 71 ans=max(ans,p); 72 printf("%.10f\n",ans); 73 74 } 75 76 77 return 0; 78 }