cogs1882 单选错位 概率与期望

链接:http://cogs.pro/cogs/problem/problem.php?pid=1882

题意:你涂串卡了,求你期望对了几个。

妈的水题两分钟……

首先,假rand()出来的数组,a[i]<=a[i+1]的话,a[i]答案一定在a[i+1]中,那么第i+1个对的概率就为1/a[i+1];

反之,a[i]不一定在a[i+1]中,在的概率为a[i+1]/a[i],对的概率为1/a[i]。

闹了半天就是取较大的那个的倒数(╯‵□′)╯︵┻━┻吔屎啦(╯‵□′)╯︵┻━┻

 1 #include<iostream>
 2 #include<cstdio>
 3 #include<algorithm>
 4 #include<cstring>
 5 using namespace std;
 6 const int maxn=10000005;
 7 int n,A,B,C,a[maxn];
 8 int haha()
 9 {
10     freopen("nt2011_exp.in","r",stdin);
11     freopen("nt2011_exp.out","w",stdout);
12     scanf("%d%d%d%d%d",&n,&A,&B,&C,a+1);
13     for (int i=2;i<=n;i++)
14         a[i] = ((long long)a[i-1] * A + B) % 100000001;
15     for (int i=1;i<=n;i++)a[i] = a[i] % C + 1;
16     double ans=0;
17     for(int i=1;i<n;i++)ans+=1/(double)max(a[i],a[i+1]);
18     ans+=1/(double)max(a[1],a[n]);
19     printf("%.3lf\n",ans);
20 }
21 int sb=haha();
22 int main(){;}
cogs1882

 

posted @ 2017-07-15 16:22  ccc000111  阅读(203)  评论(0编辑  收藏  举报