UVALive 6887 Book Club

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 ;
}