P2017 [USACO09DEC]Dizzy Cows G

P2017 [USACO09DEC]Dizzy Cows G

首先对有向边，拓扑排序，得到一个顺序，对于无向边，将顺序调整为拓扑序由小到大即可。

查看代码

#include <iostream>
#include <cstdio>
#include <queue>
using namespace std;
const int N = 1e5 + 10, M = 1e5 + 10;
queue <int> q;
int n, p1, p2, stmp, din[N], order[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 >> p1 >> p2;
    for (int i = 1; i <= n; i++)
        hd[i] = -1;
    for (int i = 1, a, b; i <= p1; i++)
    {
        cin >> a >> b;
        add(a, b);
        din[b]++;
    }
    for (int i = 1; i <= n ;i++)
        if (!din[i])
            q.push(i);
    while (!q.empty())
    {
        int t = q.front();
        q.pop();
        order[t] = ++ stmp;
        for (int i = hd[t]; ~i; i = nxt[i])
        {
            din[edg[i]]--;
            if (~din[edg[i]])
                q.push(edg[i]);
        }
    }
    for (int i = 1, a, b; i <= p2; i++)
    {
        cin >> a >> b;
        if (order[a] > order[b])
            swap(a, b);
        cout << a << ' ' << b << endl;
    }
    return 0;
}