hdu 4576 概率dp **

题意:Michael has a telecontrol robot. One day he put the robot on a loop with n cells. The cells are numbered from 1 to n clockwise.

At first the robot is in cell 1. Then Michael uses a remote control to send m commands to the robot. A command will make the robot walk some distance. Unfortunately the direction part on the remote control is broken, so for every command the robot will chose a direction(clockwise or anticlockwise) randomly with equal possibility, and then walk w cells forward.
Michael wants to know the possibility of the robot stopping in the cell that cell number >= l and <= r after m commands.

链接:点我

时间上卡的有点紧

 1 #include<cstdio>
 2 #include<iostream>
 3 #include<algorithm>
 4 #include<cstring>
 5 #include<cmath>
 6 #include<queue>
 7 #include<map>
 8 using namespace std;
 9 #define MOD 1000000007
10 const int INF=0x3f3f3f3f;
11 const double eps=1e-5;
12 typedef long long ll;
13 #define cl(a) memset(a,0,sizeof(a))
14 #define ts printf("*****\n");
15 const int MAXN=220;
16 int n,m,tt;
17 double dp[2][MAXN];
18 int a[MAXN];
19 int main()
20 {
21     int i,j,k;
22     #ifndef ONLINE_JUDGE
23     freopen("1.in","r",stdin);
24     #endif
25     int x,l,r;
26     while(scanf("%d%d%d%d",&n,&m,&l,&r)!=EOF)
27     {
28         if(n==0&&m==0&&l==0&&r==0)break;
29         dp[0][0]=1;
30         for(i=1;i<=n;i++)    dp[0][i]=0;
31         int now=0;
32         while(m--)
33         {
34             scanf("%d",&x);
35             for(i=0;i<n;i++)
36             {
37                 dp[now^1][i]=0;
38             }
39             for(i=0;i<n;i++)
40             {
41                 if(dp[now][i]==0)   continue;
42                 dp[now^1][((i-x)%n+n)%n]+=0.5*dp[now][i];
43                 dp[now^1][(i+x)%n]+=0.5*dp[now][i];
44             }
45             now^=1;
46         }
47         double ans=0;
48         for(i=l-1;i<r;i++)
49         {
50             ans+=dp[now][i];
51         }
52         printf("%.4lf\n",ans);
53     }
54 }