P2756 飞行员配对方案问题

#include <iostream>
#include<cmath>
#include <queue>
#include <cstring>
using namespace std;
 const int N =2e6+2,M=5e4+100;
 
 
 int g[N],a[N],L;
  
 const int inf =1e9+7;
 
 int all=1,hd[N],go[M],w[M],nxt[M];
  
 int S,T,n,m;
 int dis[M],ans=0,now[M];
  
 void add_(int x,int y,int z){
     nxt[++all]=hd[x]; hd[x]=all; go[all]=y;
     w[all]=z;
     swap(x,y);
     nxt[++all]=hd[x]; hd[x]=all; go[all]=y;
     w[all]=0;
 }
  bool bfs(){
     for(int i=0;i<M;i++)dis[i]=inf;
     queue<int> q;
     q.push(S);
     now[S]=hd[S];
     dis[S]=0;
      
     while(q.empty()==0){
         int x=q.front();
         q.pop();
         for(int i=hd[x];i;i=nxt[i]){
             int y=go[i];
             if(w[i]>0&&dis[y]==inf){
                 dis[y]=dis[x]+1;
                 now[y]=hd[y];
                 q.push(y);
                 if(y==T) return 1;
             }
         }
     }
     return 0;
 }
 int dfs(int x,int sum){
     if(x==T) return sum;
     int k,res=0;
      
     for(int i=now[x];i&& sum ;i=nxt[i]){
         now[x]=i;
         int y=go[i];
         if(w[i]>0&&(dis[y]==dis[x]+1)){
             k=dfs(y,min(sum,w[i]));
             if(k==0) dis[y]=inf;
             w[i]-=k;
             w[i^1]+=k;
             res+=k;
             sum-=k;
         }
     }
     return res;
 }
  
  
 signed main(){
 	int i,x,y;
 	
 	cin>>m>>n;
 	S=0,T=n+1;
 	for(i=1;i<=m;i++) add_(S,i,1); 
 	while(cin>>x>>y,~x&&~y){
 		add_(x,y,1); 
 	}
 	for(i=m+1;i<=n;i++) add_(i,T,1); 
 	
 	int ans =0;
 	while(bfs()) ans+=dfs(S,inf) ;
 	cout<<ans<<endl;
 	
 	for(i=1;i<=m;i++)
 	 for(int k=hd[i];k;k=nxt[k]){
 	 	int y=go[k]; 
 	 	if(y!=S&&w[k]==0&&w[k^1]){
 	 		cout<<i<<' '<<y<<endl;
 	 	} 
 	 }
 }