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[100],s,k1,c,f1[100],f[100][100][100];
int main()
{
    cin>>n>>k1>>c;
    for (int i=1;i<=n;i++)
     {
      cin>>a[i];
      s+=a[i];
     }
    for (int i=1;i<=n;i++) f1[i]=a[i]/s;
    for (int i=1;i<=n;i++)
     for (int j=1;j<=n;j++)
      for (int x=0;x<=n-1;x++)f[i][j][x]=210000000;
    for (int i=1;i<=n+1;i++)
     for (int x=0;x<=n-1;x++)f[i][i][x]=(k1*(x+1)+c)*f1[i];
    for (int i=1;i<=n+1;i++)
     for (int x=0;x<=n;x++)f[i][i-1][x]=0;
    for (int len=2;len<=n;len++)
     for (int i=1;i<=n-len+1;i++)
      {
        int j=i+len-1;
        for (int x=0;x<=n-1;x++)
        for (int k=i;k<=j;k++)
         f[i][j][x]=min(f[i][j][x],f[i][k-1][x+1]+f[k+1][j][x+1]+(k1*(x+1)+c)*f1[k]);
      }
    printf("%.3f",f[1][n][0]);
    return 0;
}