Title
Give you a graph, there are undirected edges and directed edges, ask you to change the undirected edges into directed edges, and ask if there is a kind of change. Yes, the graph has no loops. If yes, and output this change Scheme, if there is no output no.
answer
In fact, I had some ideas when playing games, but there was no time. Just topsort directly. When sorting, if you encounter an undirected edge, just add a direction to it. See the code for details.
#include <bits/stdc++.h>
#define ll long long
#define pi pair<int,int>
#define mk make_pair
#define pb push_back
using namespace std;
const int maxn = 2e5+10;
vector<int>G[maxn];
struct node{
int op,u,v;
}a[maxn];
int in[maxn];
map<pi,int>mp,vis;
bool topsort(int n)
{
queue<int>q;
for(int i=1;i<=n;i++)if(in[i] == 0)q.push(i);
while(q.size())
{
int u = q.front();
q.pop();
for(int i=0;i<G[u].size();i++)
{
int v = G[u][i];
if(vis[mk(v,u)] == 2)continue;
if(mp[mk(u,v)]) {
vis[mk(u,v)] = 2;
continue;
}
if((--in[v]) == 0)q.push(v);
}
}
for(int i=1;i<=n;i++)if(in[i])return false;
return true;
}
int main()
{
int T;
cin >> T;
while(T--)
{
int n,m;
scanf("%d%d",&n,&m);
mp.clear();
vis.clear();
for(int i=1;i<=n;i++)in[i] = 0, G[i].clear();
for(int i=1;i<=m;i++)
{
scanf("%d%d%d",&a[i].op,&a[i].u,&a[i].v);
if(a[i].op == 1)
{
in[a[i].v]++;
G[a[i].u].pb(a[i].v);
}
else
{
mp[mk(a[i].u , a[i].v)] = 1;
mp[mk(a[i].v , a[i].u)] = 1;
G[a[i].u].pb(a[i].v);
G[a[i].v].pb(a[i].u);
}
}
bool ok = topsort(n);
if(ok)
{
puts("YES");
for(int i=1;i<=m;i++)
{
if(a[i].op == 1)printf("%d %d\n",a[i].u,a[i].v);
else
{
if(vis[mk(a[i].u,a[i].v)] == 2)printf("%d %d\n",a[i].u,a[i].v);
else if(vis[mk(a[i].v,a[i].u)] == 2)printf("%d %d\n",a[i].v,a[i].u);
}
}
}
else puts("NO");
}
return 0;
}