码迷,mamicode.com
首页 > 其他好文 > 详细

xtu DP Training B. Collecting Bugs

时间:2014-08-02 20:36:46      阅读:258      评论:0      收藏:0      [点我收藏+]

标签:style   blog   http   color   java   os   io   for   

B. Collecting Bugs

Time Limit: 10000ms
Memory Limit: 64000KB
64-bit integer IO format: %lld      Java class name: Main
Special Judge
 
Ivan is fond of collecting. Unlike other people who collect post stamps, coins or other material stuff, he collects software bugs. When Ivan gets a new program, he classifies all possible bugs into n categories. Each day he discovers exactly one bug in the program and adds information about it and its category into a spreadsheet. When he finds bugs in all bug categories, he calls the program disgusting, publishes this spreadsheet on his home page, and forgets completely about the program. 
Two companies, Macrosoft and Microhard are in tight competition. Microhard wants to decrease sales of one Macrosoft program. They hire Ivan to prove that the program in question is disgusting. However, Ivan has a complicated problem. This new program has s subcomponents, and finding bugs of all types in each subcomponent would take too long before the target could be reached. So Ivan and Microhard agreed to use a simpler criteria --- Ivan should find at least one bug in each subsystem and at least one bug of each category. 
Macrosoft knows about these plans and it wants to estimate the time that is required for Ivan to call its program disgusting. It‘s important because the company releases a new version soon, so it can correct its plans and release it quicker. Nobody would be interested in Ivan‘s opinion about the reliability of the obsolete version. 
A bug found in the program can be of any category with equal probability. Similarly, the bug can be found in any given subsystem with equal probability. Any particular bug cannot belong to two different categories or happen simultaneously in two different subsystems. The number of bugs in the program is almost infinite, so the probability of finding a new bug of some category in some subsystem does not reduce after finding any number of bugs of that category in that subsystem. 
Find an average time (in days of Ivan‘s work) required to name the program disgusting.
 

Input

Input file contains two integer numbers, n and s (0 < n, s <= 1 000).
 

Output

Output the expectation of the Ivan‘s working days needed to call the program disgusting, accurate to 4 digits after the decimal point.
 

Sample Input

1 2

Sample Output

3.0000

解题:概率dp,求期望逆着推。

一个软件有s个子系统,会产生n种bug
某人一天发现一个bug,这个bug属于一个子系统,属于一个分类
每个bug属于某个子系统的概率是1/s,属于某种分类的概率是1/n
问发现n种bug,每个子系统都发现bug的天数的期望。

dp[i][j]表示已经找到i种bug,j个系统的bug,达到目标状态的天数的期望
dp[n][s]=0;要求的答案是dp[0][0];
dp[i][j]可以转化成以下四种状态:
dp[i][j],发现一个bug属于已经有的i个分类和j个系统。概率为(i/n)*(j/s);
dp[i][j+1],发现一个bug属于已有的分类,不属于已有的系统.概率为 (i/n)*(1-j/s);
dp[i+1][j],发现一个bug属于已有的系统,不属于已有的分类,概率为 (1-i/n)*(j/s);
dp[i+1][j+1],发现一个bug不属于已有的系统,不属于已有的分类,概率为 (1-i/n)*(1-j/s);
整理便得到转移方程


bubuko.com,布布扣
 1 #include <iostream>
 2 #include <cstdio>
 3 #include <cstring>
 4 #include <cstdlib>
 5 #include <vector>
 6 #include <climits>
 7 #include <algorithm>
 8 #include <cmath>
 9 #define LL long long
10 #define INF 0x3f3f3f
11 using namespace std;
12 const int maxn = 1010;
13 double dp[maxn][maxn];
14 int main(){
15     int n,s,i,j;
16     while(~scanf("%d%d",&n,&s)){
17         dp[n][s] = 0;
18         for(i = n; i >= 0; i--){
19             for(j = s; j >= 0; j--){
20                 if(i == n && j == s) continue;
21                  dp[i][j]=(i*(s-j)*dp[i][j+1]+(n-i)*j*dp[i+1][j]+(n-i)*(s-j)*dp[i+1][j+1]+n*s)/(n*s-i*j);
22             }
23         }
24         printf("%.4f\n",dp[0][0]);
25     }
26     return 0;
27 }
View Code

 

xtu DP Training B. Collecting Bugs,布布扣,bubuko.com

xtu DP Training B. Collecting Bugs

标签:style   blog   http   color   java   os   io   for   

原文地址:http://www.cnblogs.com/crackpotisback/p/3887268.html

(0)
(0)
   
举报
评论 一句话评论(0
登录后才能评论!
© 2014 mamicode.com 版权所有  联系我们:gaon5@hotmail.com
迷上了代码!