题干：
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 2Sample Output
3.0000题目大意：
一个软件有s个子系统，会产生n种bug。某人一天一定会发现一个bug，这个bug属于某种bug，发生在某个子系统中。求找到所有的n种bug，且每个子系统都找到bug，这样所要的天数的期望。
解题报告：
需要注意的是：bug的数量是无穷大的，所以发现一个bug，出现在某个子系统的概率是1/s，属于某种类型的概率是1/n。
又有：期望可以分解成多个子期望的加权和，权为子期望发生的概率，即 E(aA+bB+...) = aE(A) + bE(B) +...
以下题解来自kuangbin：
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
注意不能用注释的那个代码，因为没有整理，所以等号左侧和右侧都有dp[i][j]，这样是不行的。
AC代码：
#include<cstdio>
#include<iostream>
#include<algorithm>
#include<queue>
#include<map>
#include<vector>
#include<set>
#include<string>
#include<cmath>
#include<cstring>
#define F first
#define S second
#define ll long long
#define pb push_back
#define pm make_pair
using namespace std;
typedef pair<int,int> PII;
const int MAX = 2e5 + 5;
int n,s;
double dp[1555][1555];
int main()
{cin>>n>>s;dp[n][s]=0;for(int i = n; i>=0; i--) {for(int j = s; j>=0; j--) {if(i == n && j == s) continue;
// dp[i][j] = 1 + dp[i+1][j] * (n-i)*j/n/s + dp[i][j] * i*j/n/s + dp[i][j+1] *i*(s-j)/n/s + dp[i+1][j+1]*(n-i)*(s-j)/n/s;dp[i][j]=((n-i)*j*dp[i+1][j]+(n-i)*(s-j)*dp[i+1][j+1]+i*(s-j)*dp[i][j+1]+n*s)/(n*s-i*j);}}printf("%.4f\n",dp[0][0]);return 0 ;
}
)
