13年山东省赛 The number of steps（概率dp水题）

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The number of steps

Time Limit: 1 Sec  Memory Limit: 128 M

Description

Mary stands in a strange maze, the maze looks like a triangle（the first layer have one room，the second layer have two rooms，the third layer have three rooms …）. Now she stands at the top point(the first layer), and the KEY of this maze is in the lowest layer’s leftmost room. Known that each room can only access to its left room and lower left and lower right rooms .If a room doesn’t have its left room, the probability of going to the lower left room and lower right room are a and b (a + b = 1 ). If a room only has it’s left room, the probability of going to the room is 1. If a room has its lower left, lower right rooms and its left room, the probability of going to each room are c, d, e (c + d + e = 1). Now , Mary wants to know how many steps she needs to reach the KEY. Dear friend, can you tell Mary the expected number of steps required to reach the KEY?

 

 

Input

There are no more than 70 test cases. 

In each case , first Input a positive integer n(0<n<45), which means the layer of the maze, then Input five real number a, b, c, d, e. (0<=a,b,c,d,e<=1, a+b=1, c+d+e=1). 

The input is terminated with 0. This test case is not to be processed.

 

Output

Please calculate the expected number of steps required to reach the KEY room, there are 2 digits after the decimal point.

 

Sample Input

3

0.3 0.7

0.1 0.3 0.6

0 

Sample Output

3.41

概率dp大水题。。。

 1 #include <iostream>

 2 #include <sstream>

 3 #include <ios>

 4 #include <iomanip>

 5 #include <functional>

 6 #include <algorithm>

 7 #include <vector>

 8 #include <string>

 9 #include <list>

10 #include <queue>

11 #include <deque>

12 #include <stack>

13 #include <set>

14 #include <map>

15 #include <cstdio>

16 #include <cstdlib>

17 #include <cmath>

18 #include <cstring>

19 #include <climits>

20 #include <cctype>

21 using namespace std;

22 #define XINF INT_MAX

23 #define INF 0x3FFFFFFF

24 #define MP(X,Y) make_pair(X,Y)

25 #define PB(X) push_back(X)

26 #define REP(X,N) for(int X=0;X<N;X++)

27 #define REP2(X,L,R) for(int X=L;X<=R;X++)

28 #define DEP(X,R,L) for(int X=R;X>=L;X--)

29 #define CLR(A,X) memset(A,X,sizeof(A))

30 #define IT iterator

31 typedef long long ll;

32 typedef pair<int,int> PII;

33 typedef vector<PII> VII;

34 typedef vector<int> VI;

35 double dp[110][110];

36 int main()

37 {

38     ios::sync_with_stdio(false);

39     int n;

40     while(cin>>n&&n){

41         double a,b,c,d,e;

42         cin>>a>>b>>c>>d>>e;

43         CLR(dp,0);

44         for(int i=n;i;i--){

45             for(int j = n+1-i;j<=n;j++){

46                 if(i==n&&j==(n+1-i))continue;

47                 else if(i==n)dp[i][j]=dp[i][j-1]+1.0;

48                 else if(j==(n+1-i))dp[i][j]=a*dp[i+1][j-1]+b*dp[i+1][j]+1.0;

49                 else dp[i][j]=c*dp[i+1][j-1]+d*dp[i+1][j]+e*dp[i][j-1]+1.0;

50             }

51         }

52         cout<<fixed<<setprecision(2)<<dp[1][n]<<endl;

53     }

54     return 0;

55 }