SGU 495 Kids and prizes（概率题，二项分布）

495. Kids and Prizes

Time limit per test: 0.25 second(s)

Memory limit: 262144 kilobytes
input: standard

output: standard

ICPC (International Cardboard Producing Company) is in the business of producing cardboard boxes. Recently the company organized a contest for kids for the best design of a cardboard box and selected
M winners. There are N prizes for the winners, each one carefully packed in a cardboard box (made by the ICPC, of course). The awarding process will be as follows:
All the boxes with prizes will be stored in a separate room.
The winners will enter the room, one at a time.
Each winner selects one of the boxes.
The selected box is opened by a representative of the organizing committee.
If the box contains a prize, the winner takes it.
If the box is empty (because the same box has already been selected by one or more previous winners), the winner will instead get a certificate printed on a sheet of excellent cardboard (made by ICPC, of course).

Whether there is a prize or not, the box is re-sealed and returned to the room.

The management of the company would like to know how many prizes will be given by the above process. It is assumed that each winner picks a box at random and that all boxes are equally likely to be picked. Compute the mathematical expectation of the number
of prizes given (the certificates are not counted as prizes, of course).

[align=left]Input[/align]
The first and only line of the input file contains the values of N and
M (

).

[align=left]Output[/align]
The first and only line of the output file should contain a single real number: the expected number of prizes given out. The answer is accepted as correct if either the absolute or the relative error is less than or equal to 10-9.

解题思路：

逆向思维，每个礼物不被选中的概率为((n-1) / n)^m;

每次是独立的，符合二项分布，二项分布的期望公式为n * p;

#include <iostream>
#include <cstring>
#include <cstdlib>
#include <cstdio>
#include <cmath>
#include <vector>
#include <queue>
#include <stack>
#include <set>
#include <map>
#define LL long long
using namespace std;
const int MAXN = 1000 + 10;
int main()
{
double n, m;
while(cin>>n>>m)
{
double tmp = pow((n-1) / n, m);
double ans = n - n * tmp;
printf("%.10lf\n", ans);
}
return 0;
}