P2999 [USACO10NOV]Chocolate Milk S

按照题意拓扑排序即可。
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#include <iostream>
#include <cstdio>
#include <queue>
using namespace std;
const int N = 1e5 + 10, M = 1e5 + 10;
bool vis[N];
int n, cnt, din[N], dout[N], flow[N];
int idx = -1, hd[N], nxt[M], edg[M];
void add (int a, int b)
{
    nxt[++idx] = hd[a];
    hd[a] = idx;
    edg[idx] = b;
}
int main ()
{
    cin >> n;
    for (int i = 1; i <= n; i++)
        hd[i] = -1;
    for (int i = 1, a, b; i < n; i++)
    {
        cin >> a >> b;
        add(a, b);
        din[b]++;
        dout[a]++;
    }
    queue <int> q;
    for (int i = 1; i <= n; i++)
        if (!din[i])
        {
            cnt++;
            flow[i] = 1;
            vis[i] = true;
            q.push(i);
        }
    while (!q.empty())
    {
        int t = q.front();
        q.pop();
        if (dout[t] > 1)
            continue;
        for (int i = hd[t]; ~i; i = nxt[i])
        {
            flow[edg[i]] += flow[t];
            din[edg[i]]--;
            if (!din[edg[i]])
                q.push(edg[i]);
        }
    }
    for (int i = 1; i <= n; i++)
        if (flow[i] == cnt && !vis[i])
            cout << i << endl;
    return 0;
}