codeforces 743D - Chloe and pleasant prizes(树形DP)

题意

给一颗树取出两个不相互包含的子树使权值和最大

所以对于每一个树来说，与它对应的最大的  另一棵最大权值子树    在父亲节点对应的最大权值子树    以及    父亲节点的另外孩子包含的最大子树上（感觉有点绕==

#include <cstdio>
#include <iostream>
#include <algorithm>
#include <cstring>
#define F    first
#define S     second
#define mem(a,b)    memset(a,b,sizeof(a))
using namespace std;
#define INF        0x7ffffffffffffff
typedef long long int LL;
#define MAX_N        200005
LL w[MAX_N];
struct no
{
    int v,nexr;
}ed[MAX_N*3];
int stu[MAX_N];
int col = 0;
void add(int u , int v){
    ed[col].v = v;
    ed[col].nexr = stu[u];
    stu[u] = col++;
}
pair<LL,pair<LL,LL> > ans[MAX_N];
bool flag[MAX_N];
int set[MAX_N];
int cnt[MAX_N];
void dfs(int u){
    flag[u] = true;
    ans[u].F = w[u];
    for (int i = stu[u]; i != -1  ; i = ed[i].nexr)
    {
        int v = ed[i].v;
        if(flag[v])
            continue;
        dfs(v);
        ans[u].F+=ans[v].F;
        if(ans[v].S.F>ans[u].S.F||ans[v].F>ans[u].S.F){
            set[u]  = v;
            ans[u].S.S = ans[u].S.F;
            ans[u].S.F = max(ans[v].S.F, ans[v].F);
        }
        else if(ans[v].S.F>ans[u].S.S||ans[v].F>ans[u].S.S){
            ans[u].S.S = max(ans[v].S.F,ans[v].F);
        }
    }
}
LL res[MAX_N];
void dfs2(int u){
    flag[u] = true;
    for (int i = stu[u]; i != -1  ; i = ed[i].nexr)
    {
        int v = ed[i].v;
        if(flag[v])
            continue;
        if(set[u] != v)
            res[v] = max (ans[u].S.F,res[u]);
        else
            res[v] = max (ans[u].S.S,res[u]);
        dfs2(v);
    }
}

int main(int argc, char const *argv[])
{
    int n;
    mem(stu,-1);
    scanf("%d",&n);
    for (int i = 1; i <= n ; ++i){
        scanf("%I64d",&w[i]);
        ans[i].F = ans[i].S.F = ans[i].S.S = res[i] = -INF;
    }
    int u , v;
    for (int i = 1; i <= n-1 ; ++i)
    {
        scanf("%d%d",&u,&v);
        add(u,v);
        add(v,u);
        cnt[u]++;
        cnt[v]++;
    }
    dfs(1);
    mem(flag,false);
    dfs2(1);
    LL pri = -INF;
    for (int i = 1; i <= n ; ++i)
    {
        if(res[i]!=-INF){
            pri = max(res[i]+ans[i].F,pri);
        }
    }
    if(pri==-INF)
        printf("Impossible\n");
    else
        printf("%I64d\n",pri );
    return 0;
}