2015
07-16

# Candy

LazyChild is a lazy child who likes candy very much. Despite being very young, he has two large candy boxes, each contains n candies initially. Everyday he chooses one box and open it. He chooses the first box with probability p and the second box with probability (1 – p). For the chosen box, if there are still candies in it, he eats one of them; otherwise, he will be sad and then open the other box.
He has been eating one candy a day for several days. But one day, when opening a box, he finds no candy left. Before opening the other box, he wants to know the expected number of candies left in the other box. Can you help him?

There are several test cases.
For each test case, there is a single line containing an integer n (1 ≤ n ≤ 2 × 105) and a real number p (0 ≤ p ≤ 1, with 6 digits after the decimal).
Input is terminated by EOF.

There are several test cases.
For each test case, there is a single line containing an integer n (1 ≤ n ≤ 2 × 105) and a real number p (0 ≤ p ≤ 1, with 6 digits after the decimal).
Input is terminated by EOF.

10 0.400000
100 0.500000
124 0.432650
325 0.325100
532 0.487520
2276 0.720000

Case 1: 3.528175
Case 2: 10.326044
Case 3: 28.861945
Case 4: 167.965476
Case 5: 32.601816
Case 6: 1390.500000

对于第一个数据溢出的问题，可以这样解决。因为组合数公式为：
C(n,m) = n!/(m!(n-m)!)

为了避免直接计算n的阶乘，对公式两边取对数，于是得到：
ln(C(n,m)) = ln(n!)-ln(m!)-ln((n-m)!)

进一步化简得到：

这样我们就把连乘转换为了连加，因为ln(n)总是很小的，所以上式很难出现数据溢出。

为了解决第二个效率的问题，我们对上式再做一步化简。上式已经把连乘法变成了求和的线性运算，也就是说，上式已经极大地简化了计算的复杂度，但是还可以进一步优化。从上式中，我们很容易看出右边的3项必然存在重复的部分。现在我们把右边第一项拆成两部分：

这样，上式右边第一项就可以被抵消掉，于是得到：

上式直接减少了2m次对数计算及求和运算。但是这个公式还可以优化。对于上面公式里的求和，当m<n/2时，n-m是一个很大的数，但是当m>n/2时，n-m就会小很多。我们知道：
C(n,m) = C(n,n-m)

那么通过这个公式，我们可以把小于n/2的m变为大于n/2的n-m再进行计算，结果是一样的，但是却能减少计算量。

当计算出ln(C(n,m))后，只需要取自然对数，就可以得到组合数：
C(n,m) = exp(ln(C(n,m)))

这样就完成了组合数的计算。

用这种方法计算组合数，如果只计算ln(C(n,m))的话，n可以取到整型数据的极限值65535，

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <string>
#include <vector>
#include <cmath>
using namespace std;
double f[400008];
double C_m_n(int m,int n)
{

return f[m]-f[n]-f[m-n];
}
int main()
{
f[0]=0;
for(int i=1;i<=400006;i++)
f[i]+=f[i-1]+log(i*1.0);
double sum,p;
int n,test=0;
while(cin>>n>>p)
{
sum=0;
printf("Case %d: ",++test);
for(int k=0;k<=n-1;k++)
{
sum+=(n-k)*(exp((C_m_n(n+k,k))+(n+1)*log(p)+k*log(1-p))+exp((C_m_n(n+k,k))+(n+1)*log(1-p)+k*log(p)));
}
printf("%.6f\n",sum);
}
}