【洛谷2756】飞行员配对方案问题

题面

https://www.luogu.org/problem/P2756

题解

#include<cstdio>
#include<cstring>
#include<iostream>
#include<vector>
#include<queue>
#define ri register int
#define N 300
#define INF 1000000007
#define M 25000
using namespace std;

int n,m;

struct Dinic {
  vector<int> ed[N];
  int w[M],to[M],d[N],cur[N];
  int cnt;
  void init() {cnt=-1;}
  void add_edge(int a,int b,int c) {
    ed[a].push_back(++cnt); w[cnt]=c; to[cnt]=b;
    ed[b].push_back(++cnt); w[cnt]=0; to[cnt]=a;
  }
  bool bfs() {
    memset(d,0x3f,sizeof(d));
    queue<int> q;
    d[0]=0;
    q.push(0);
    while (!q.empty()) {
      int x=q.front(); q.pop();
      //cout<<x<<endl;
      for (ri i=0;i<ed[x].size();i++) {
        int e=ed[x][i];
        if (!w[e]) continue;
        if (d[x]+1<d[to[e]]) {
          d[to[e]]=d[x]+1;
          q.push(to[e]);
        }
      }
    }
    return d[n+m+1]<=N;
  }
  
  int dfs(int x,int limit) {
    if (!limit || x==n+m+1) return limit;
    int tot=0;
    for (ri &i=cur[x];i<ed[x].size();i++) {
      int e=ed[x][i];
      if (w[e] && d[to[e]]==d[x]+1) {
        int f=dfs(to[e],min(limit,w[e]));
        if (!f) continue;
        w[e]-=f; w[1^e]+=f; tot+=f; limit-=f;
        if (!limit) return tot;
      }
    }
    return tot;
  }
  
  int solve() {
    int ret=0;
    while (bfs()) {
      memset(cur,0,sizeof(cur));
      ret+=dfs(0,INF);
    }
    return ret;
  }
} zyx;

int main() {
  int u,v;
  scanf("%d %d",&n,&m);
  zyx.init();
  while (1) {
    scanf("%d %d",&u,&v);
    if (u==-1 && v==-1) break;
    zyx.add_edge(u,v,1);
  }
  for (ri i=1;i<=n;i++) zyx.add_edge(0,i,1);
  for (ri i=n+1;i<=n+m;i++) zyx.add_edge(i,n+m+1,1);
  printf("%d
",zyx.solve());
  for (ri i=0;i<=zyx.cnt;i+=2) {
    if (!zyx.w[i] && zyx.to[i]<=n+m && zyx.to[i]>=n+1) printf("%d %d
",zyx.to[i^1],zyx.to[i]);
  }
  return 0;
}