BZOJ1261: [SCOI2006]zh_tree

题目：http://www.lydsy.com/JudgeOnline/problem.php?id=1261

区间dp，f[l][r]=min(f[l][now-1]+f[now+1][r]+c*f[i]),然后最后再加上k*(sum[r]-sum[l-1])/sum[n]

#include<cstring>
#include<cstdio>
#include<iostream>
#include<algorithm>
#include<cmath>
#include<queue>
#define rep(i,l,r) for (int i=l;i<=r;i++)
#define down(i,l,r) for (int i=l;i>=r;i--)
#define clr(x,y) memset(x,y,sizeof(x))
#define inf int(1e9)
#define maxn 31
#define mm 30031
using namespace std;
int a[maxn],s,n;
double sum[maxn],f[maxn],k,c,d[maxn][maxn];
int vis[maxn][maxn];
int read(){
    int x=0,f=1; char ch=getchar();
    while (!isdigit(ch)) {if (ch=='-') f=-1; ch=getchar();}
    while (isdigit(ch)) {x=x*10+ch-'0'; ch=getchar();}
    return x*f;
}
double dfs(int l,int r){
    if (l>r) return 0;
    if (vis[l][r]) return d[l][r];
    if (!vis[l][r]) vis[l][r]=1;
    d[l][r]=1e20;
    rep(i,l,r) d[l][r]=min(d[l][r],dfs(l,i-1)+dfs(i+1,r)+c*(sum[i]-sum[i-1])/sum[n]);
    d[l][r]+=k*(sum[r]-sum[l-1])/sum[n];
    return d[l][r];
}
int main(){
    n=read(); scanf("%lf%lf",&k,&c);
    rep(i,1,n) {
        double x;
        scanf("%lf",&x); sum[i]=sum[i-1]+x;
    } 
    dfs(1,n);
    printf("%.3lf\n",d[1][n]);
    return 0;
}