树形DP的另类解法 ---- toposort

#include<bits/stdc++.h>
using namespace std;
#define LL long long
#define pb push_back
#define F(i,a,b) for(int i=a;i<=b;i++)
const int N = 6005;
vector <int> e[N];
queue <int> q;
int r[N],o[N],h[N],f[N][2],root,x,y,n;
int toposort()
{
    F(i,1,n) if(!r[i]) q.push(i);
    while(q.size())
    {
        int x = q.front();
        q.pop();
        for(int y:e[x])
        {
            f[y][0] += max(f[x][0],f[x][1]);
            f[y][1] += f[x][0];
            if(--r[y] == 0) q.push(y);
        }
    }
    return max(f[root][0],f[root][1]);
}
int main()
{
    cin>>n;
    F(i,1,n) cin>>f[i][1];
    F(i,1,n-1)
    {
        cin>>x>>y;
        e[x].pb(y);
        o[x]++; r[y]++;
    }
    F(i,1,n) if(!o[i]) root = i;
    cout << toposort() << '\n';
    return 0;
}