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=,f=;char c=getchar();
    while (c<''||c>'') {if (c=='-') f=-;c=getchar();}
    while (c>=''&&c<='') x=(x<<)+(x<<)+(c^),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=,t=;
bool isleaf[N],flag[N];
int L[N<<],R[N<<];
ll deep[N],tree[N<<],lazy[N<<],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]=;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<<],tree[k<<|]);}
void down(int k)
{
    tree[k<<]+=lazy[k],tree[k<<|]+=lazy[k];
    lazy[k<<]+=lazy[k],lazy[k<<|]+=lazy[k];
    lazy[k]=;
}
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>>;
    build(k<<,l,mid);
    build(k<<|,mid+,r);
    up(k);
}
int query(int k)
{
    if (L[k]==R[k]) return L[k];
    if (lazy[k]) down(k);
    if (tree[k<<]>tree[k<<|]) return query(k<<);
    else return query(k<<|);
}
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]>>;
    if (r<=mid) add(k<<,l,r,x);
    else if (l>mid) add(k<<|,l,r,x);
    else add(k<<,l,mid,x),add(k<<|,mid+,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=;i<=n;i++) a[i]=read(),isleaf[i]=;
    for (int i=;i<n;i++)
    {
        int x=read(),y=read();
        addedge(x,y);fa[y]=x;isleaf[x]=;
    }
    dfs();
    build(,,n);
    flag[]=;
    while (m--)
    {
        if (tree[]<=) break;
        ans+=tree[];
        int x=dfn[query()];
        while (!flag[x])
        {
            flag[x]=;
            add(,id[x],id[x]+size[x]-,-a[x]);
            x=fa[x];
        }
    }
    cout<<ans;
    return ;
}