D. Wilbur and Trees

Time Limit: 20 Sec

Memory Limit: 256 MB

題目連接

http://codeforces.com/contest/596/problem/D

Description

Wilbur the pig really wants to be a beaver, so he decided today to pretend he is a beaver and bite at trees to cut them down.

There are n trees located at various positions on a line. Tree i is located at position xi. All the given positions of the trees are distinct.

The trees are equal, i.e. each tree has height h. Due to the wind, when a tree is cut down, it either falls left with probability p, or falls right with probability 1?-?p. If a tree hits another tree while falling, that tree will fall in the same direction as the tree that hit it. A tree can hit another tree only if the distance between them is strictly less than h.

For example, imagine there are 4 trees located at positions 1, 3, 5 and 8, while h?=?3 and the tree at position 1 falls right. It hits the tree at position 3 and it starts to fall too. In it's turn it hits the tree at position 5 and it also starts to fall. The distance between 8 and 5 is exactly 3, so the tree at position 8 will not fall.

As long as there are still trees standing, Wilbur will select either the leftmost standing tree with probability 0.5 or the rightmost standing tree with probability 0.5. Selected tree is then cut down. If there is only one tree remaining, Wilbur always selects it. As the ground is covered with grass, Wilbur wants to know the expected total length of the ground covered with fallen trees after he cuts them all down because he is concerned about his grass-eating cow friends. Please help Wilbur.

Input

The first line of the input contains two integers, n (1?≤?n?≤?2000) and h (1?≤?h?≤?108) and a real number p (0?≤?p?≤?1), given with no more than six decimal places.

The second line of the input contains n integers, x1,?x2,?...,?xn (?-?108?≤?xi?≤?108) in no particular order.

Output

Print a single real number?— the expected total length of the ground covered by trees when they have all fallen down. Your answer will be considered correct if its absolute or relative error does not exceed?10?-?6.

Namely: let's assume that your answer is?a, and the answer of the jury is?b. The checker program will consider your answer correct, if?.

Sample Input

2 2 0.500000
1 2

Sample Output

3.250000000

HINT

?

題意

?在一個平面上有n棵樹，每棵樹高為h，你是一個伐木工人，每次有1/2的概率選擇砍掉最左邊或者最右邊的樹

樹也有p的概率向左倒，(1-p)的概率向右倒?

樹如果倒下的時候，壓中了別的樹，那么那棵樹也會跟著倒下

然后問你，最后倒下的樹的期望長度總和是多少

題解：

區間dp，dfs(l,r,f1,f2)

f1表示這個l-1這棵樹是否倒向了右邊，f2表示r+1這棵樹是否倒向了左邊

說是dp，實質上就是dfs，直接枚舉所有的情況暴力dfs就好了

代碼

#include<iostream> #include<stdio.h> #include<algorithm> using namespace std; #define maxn 2005 const int inf = 1e9; double dp[maxn][maxn][2][2]; int vis[maxn][maxn][2][2]; int n; double h,p; int v[maxn]; int dl[maxn],dr[maxn]; double dfs(int l,int r,int f1,int f2) {//cout<<l<<" "<<r<<" "<<f1<<" "<<f2<<endl;if(vis[l][r][f1][f2])return dp[l][r][f1][f2];if(l>r)return 0;vis[l][r][f1][f2]=1;double ans = dp[l][r][f1][f2];ans+=p*0.5*(min(h*1.00,v[l]-v[l-1]-f1*h)+dfs(l+1,r,0,f2));//最左邊那個朝左邊倒ans+=(1-p)*0.5*(min(h*1.0,v[r+1]-v[r]-f2*h)+dfs(l,r-1,f1,0));//最右邊那個朝右邊倒int L = dr[l];//左邊向右邊倒int R = dl[r];//右邊向左邊倒if(R<=l)ans+=p*0.5*(v[r]-v[l]+min(h,v[l]-v[l-1]-f1*h));else ans+=p*0.5*(v[r]-v[R]+h+dfs(l,R-1,f1,1));if(L>=r)ans+=(1-p)*0.5*(v[r]-v[l]+min(h,v[r+1]-v[r]-f2*h));else ans+=(1-p)*0.5*(v[L]-v[l]+h+dfs(L+1,r,1,f2));dp[l][r][f1][f2] = ans;return ans; } int main() {scanf("%d%lf",&n,&h);scanf("%lf",&p);for(int i=1;i<=n;i++)scanf("%d",&v[i]);sort(v+1,v+1+n);v[n+1]=inf;v[0]=-inf;dr[n]=n;dl[1]=1;for(int i=n-1;i>=1;i--){if(v[i+1]-v[i]<h)dr[i]=dr[i+1];else dr[i]=i;}for(int i=2;i<=n;i++){if(v[i]-v[i-1]<h)dl[i]=dl[i-1];else dl[i]=i;}//cout<<dfs(1,n,0,0)<<endl;printf("%.15f\n",dfs(1,n,0,0)); }

?

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