bzoj1261

题解：

看到了树

很明显就是树形dp吗

然后随便yy一下方程就好了

代码：

#include<bits/stdc++.h>
using namespace std;
int n,j;
double a[],s,k1,c,f1[],f[][][];
int main()
{
    cin>>n>>k1>>c;
    for (int i=;i<=n;i++)
     {
      cin>>a[i];
      s+=a[i];
     }
    for (int i=;i<=n;i++) f1[i]=a[i]/s;
    for (int i=;i<=n;i++)
     for (int j=;j<=n;j++)
      for (int x=;x<=n-;x++)f[i][j][x]=;
    for (int i=;i<=n+;i++)
     for (int x=;x<=n-;x++)f[i][i][x]=(k1*(x+)+c)*f1[i];
    for (int i=;i<=n+;i++)
     for (int x=;x<=n;x++)f[i][i-][x]=;
    for (int len=;len<=n;len++)
     for (int i=;i<=n-len+;i++)
      {
        int j=i+len-;
        for (int x=;x<=n-;x++)
        for (int k=i;k<=j;k++)
         f[i][j][x]=min(f[i][j][x],f[i][k-][x+]+f[k+][j][x+]+(k1*(x+)+c)*f1[k]);
      }
    printf("%.3f",f[][n][]);
    return ;
}