BZOJ3252 攻略（贪心+dfs序+线段树）

　　考虑贪心，每次选价值最大的链。选完之后对于链上点dfs序暴力修改子树。因为每个点最多被选一次，复杂度非常正确。

#include<iostream> 
#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<cstring>
#include<algorithm>
using namespace std;
int read()
{
    int x=0,f=1;char c=getchar();
    while (c<'0'||c>'9') {if (c=='-') f=-1;c=getchar();}
    while (c>='0'&&c<='9') x=(x<<1)+(x<<3)+(c^48),c=getchar();
    return x*f;
}
#define N 200010
#define ll long long
#define inf 1000000000000000
int n,m,a[N],p[N],fa[N],id[N],dfn[N],size[N],cnt=0,t=0;
bool isleaf[N],flag[N];
int L[N<<2],R[N<<2];
ll deep[N],tree[N<<2],lazy[N<<2],ans;
struct data{int to,nxt;
}edge[N];
void addedge(int x,int y){t++;edge[t].to=y,edge[t].nxt=p[x],p[x]=t;}
void dfs(int k)
{
    size[k]=1;deep[k]=deep[fa[k]]+a[k];
    id[k]=++cnt;dfn[cnt]=k;
    for (int i=p[k];i;i=edge[i].nxt) 
    if (edge[i].to!=fa[k]) dfs(edge[i].to),size[k]+=size[edge[i].to];
}
void up(int k){tree[k]=max(tree[k<<1],tree[k<<1|1]);}
void down(int k)
{
    tree[k<<1]+=lazy[k],tree[k<<1|1]+=lazy[k];
    lazy[k<<1]+=lazy[k],lazy[k<<1|1]+=lazy[k];
    lazy[k]=0;
}
void build(int k,int l,int r)
{
    L[k]=l,R[k]=r;
    if (l==r) {tree[k]=isleaf[dfn[l]]?deep[dfn[l]]:-inf;return;}
    int mid=l+r>>1;
    build(k<<1,l,mid);
    build(k<<1|1,mid+1,r);
    up(k);
}
int query(int k)
{
    if (L[k]==R[k]) return L[k];
    if (lazy[k]) down(k);
    if (tree[k<<1]>tree[k<<1|1]) return query(k<<1);
    else return query(k<<1|1);
}
void add(int k,int l,int r,int x)
{
    if (L[k]==l&&R[k]==r) {tree[k]+=x,lazy[k]+=x;return;}
    if (lazy[k]) down(k);
    int mid=L[k]+R[k]>>1;
    if (r<=mid) add(k<<1,l,r,x);
    else if (l>mid) add(k<<1|1,l,r,x);
    else add(k<<1,l,mid,x),add(k<<1|1,mid+1,r,x);
    up(k);
}
int main()
{
#ifndef ONLINE_JUDGE
    freopen("bzoj3252.in","r",stdin);
    freopen("bzoj3252.out","w",stdout);
    const char LL[]="%I64d\n";
#else
    const char LL[]="%lld\n";
#endif
    n=read(),m=read();
    for (int i=1;i<=n;i++) a[i]=read(),isleaf[i]=1;
    for (int i=1;i<n;i++)
    {
        int x=read(),y=read();
        addedge(x,y);fa[y]=x;isleaf[x]=0;
    }
    dfs(1);
    build(1,1,n);
    flag[0]=1;
    while (m--)
    {
        if (tree[1]<=0) break;
        ans+=tree[1];
        int x=dfn[query(1)];
        while (!flag[x])
        {
            flag[x]=1;
            add(1,id[x],id[x]+size[x]-1,-a[x]);
            x=fa[x];
        }
    } 
    cout<<ans;
    return 0;
}