bzoj 2318: Spoj4060 game with probability Problem (概率与期望DP)
2017-03-16 09:06
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2318: Spoj4060 game with probability Problem
Time Limit: 1 Sec Memory Limit: 128 MBSubmit: 362 Solved: 167
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Description
Alice和Bob在玩一个游戏。有n个石子在这里,Alice和Bob轮流投掷硬币,如果正面朝上,则从n个石子中取出一个石子,否则不做任何事。取到最后一颗石子的人胜利。Alice在投掷硬币时有p的概率投掷出他想投的一面,同样,Bob有q的概率投掷出他相投的一面。现在Alice先手投掷硬币,假设他们都想赢得游戏,问你Alice胜利的概率为多少。
Input
第一行一个正整数t,表示数据组数。对于每组数据,一行三个数n,p,q。
Output
对于每组数据输出一行一个实数,表示Alice胜利的概率,保留6位小数。Sample Input
11 0.5 0.5
Sample Output
0.666667HINT
数据范围:1<=t<=50
0.5<=p,q<=0.99999999
对于100%的数据 1<=n<=99999999
Source
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题解:概率与期望DP
这道题的n很大,理论上O(n)的是做不了的,但是这道题的精度很小,那么当n很大很大的时候,概率的波动会非常小,基本上是保持不变的,所有我们可以只做10000次,或者更少。
f[i]表示剩i颗石子,Alice是先手赢的概率
g[i]表示剩i颗石子,Alice是后手赢的概率
如果g[i-1]>f[i-1],那么说明对于i-1颗石子的情况来说,后手更有利,那么Alice一定想拿到i-1的后手权,那么他会想拿到第i颗石子,即p的概率赢的后手权。如果1-p的概率扔到了反面,那么i状态就转换成了g[i]
f[i]=p*g[i-1]+(1-p)*g[i]
对于Bob,他现在是先手,那么他一定想i-1的时候Alice是先手,所以如果扔到正面,那么i-1颗石子Alice必然是先手,否则就把i状态给了Alice
g[i]=f[i]*(1-q)+f[i-1]*q
两个方程连立就可以解出来。
如果g[i-1]<f[i-1],那么说明对于i-1颗石子的情况来说,先手更有利。
f[i]=(1-p)*g[i-1]+p*g[i]
g[i]=f[i]*q+f[i-1]*(1-q) 用与上面相同的方法分析求解
最初的状态是f[0]=0,g[0]=1;
#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cmath>
#include<cstring>
#define N 10003
using namespace std;
int n,T;
double f
,g
,p,q,q1,p1;
int main()
{
freopen("a.in","r",stdin);
freopen("my.out","w",stdout);
scanf("%d",&T);
while (T--) {
scanf("%d%lf%lf",&n,&p,&q);
n=min(n,10000);
p1=1.0-p; q1=1.0-q;
memset(f,0,sizeof(f));
memset(g,0,sizeof(g));
f[0]=0; g[0]=1;
for (int i=1;i<=n;i++)
if (g[i-1]>f[i-1])
{
double t=1.0-p1*q1;
f[i]=(g[i-1]*p+p1*q*f[i-1])/t;
g[i]=(g[i-1]*p*q1+f[i-1]*q)/t;
//cout<<f[i]<<" "<<g[i]<<endl;
}
else {
double t=1.0-p*q;
f[i]=(g[i-1]*p1+p*q1*f[i-1])/t;
g[i]=(q1*f[i-1]+p1*q*g[i-1])/t;
//cout<<f[i]<<" "<<g[i]<<endl;
}
printf("%.6lf\n",f
);
}
}
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