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 }