A - Alien's Organ

http://acm.hust.edu.cn/vjudge/contest/view.action?cid=23407#problem/A

套用泊松分布

#include <stdio.h>
#include <iostream>
#include <cmath>
using namespace std;

int t;
int n;
double k;

int main()
{
	cin >> t;
	while(t --)
	{
		cin >> n >> k;
		double temp = 1, sum=1;
		for(int i=1; i<=n ; ++i)
		{
			temp *= k;
			temp /= i;
			sum += temp;
		}
	//	cout<< exp(-la)<<endl;
		sum *= exp(-k);
		printf("%.3lf\n", sum);
	}
	return 0;
}