最大流，有向环覆盖问题。
#include<cstdio>
#include<cstring>
#include<string>
#include<cmath>
#include<vector>
#include<queue>
#include<algorithm>
using namespace std; const int maxn = + ;
const int INF = 0x7FFFFFFF;
struct Edge
{
int from, to, cap, flow;
Edge(int u, int v, int c, int f) :from(u), to(v), cap(c), flow(f){}
};
vector<Edge>edges;
vector<int>G[maxn];
bool vis[maxn];
int d[maxn];
int cur[maxn];
int n, m, s, t; void init()
{
for (int i = ; i < maxn; i++)
G[i].clear();
edges.clear();
}
void AddEdge(int from, int to, int cap)
{
edges.push_back(Edge(from, to, cap, ));
edges.push_back(Edge(to, from, , ));
int w = edges.size();
G[from].push_back(w - );
G[to].push_back(w - );
}
bool BFS()
{
memset(vis, , sizeof(vis));
queue<int>Q;
Q.push(s);
d[s] = ;
vis[s] = ;
while (!Q.empty())
{
int x = Q.front();
Q.pop();
for (int i = ; i<G[x].size(); i++)
{
Edge e = edges[G[x][i]];
if (!vis[e.to] && e.cap>e.flow)
{
vis[e.to] = ;
d[e.to] = d[x] + ;
Q.push(e.to);
}
}
}
return vis[t];
}
int DFS(int x, int a)
{
if (x == t || a == )
return a;
int flow = , f;
for (int &i = cur[x]; i<G[x].size(); i++)
{
Edge e = edges[G[x][i]];
if (d[x]+ == d[e.to]&&(f=DFS(e.to,min(a,e.cap-e.flow)))>)
{
edges[G[x][i]].flow+=f;
edges[G[x][i] ^ ].flow-=f;
flow+=f;
a-=f;
if(a==) break;
}
}
if(!flow) d[x] = -;
return flow;
}
int dinic(int s, int t)
{
int flow = ;
while (BFS())
{
memset(cur, , sizeof(cur));
flow += DFS(s, INF);
}
return flow;
} int main()
{
while(~scanf("%d%d",&n,&m)){
init();
s=;t=*n+;
for(int i=;i<=n;i++) AddEdge(s,i,);
for(int i=n+;i<=*n;i++) AddEdge(i,t,);
for(int i=;i<=m;i++)
{
int u,v;
scanf("%d%d",&u,&v);
u++;
v++;
AddEdge(u,v+n,);
}
if(dinic(s,t)==n) printf("YES\n");
else printf("NO\n");
}
return ;
}