飞行员配对方案[网络流]

传送门

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直接网络流模板

注意输出方案,我们发现,但我们简单方向边不为0时,边的两头就是选了的

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 代码

#include<bits/stdc++.h>
#define N 105
#define M N*N*4 
#define inf 0x3fffffff
using namespace std;
int first[N],next[M],to[M],w[M],tot=1;
int m,n,ed,st,x,y,dis[N],vis[N];
void add(int x,int y,int z){
	next[++tot]=first[x],first[x]=tot,to[tot]=y,w[tot]=z;
}
bool bfs(){
	memset(dis,0,sizeof(dis));
	memset(vis,0,sizeof(vis));
	queue<int> q; q.push(st); vis[st]=1;
	while(!q.empty()){
		int x=q.front(); q.pop();
		for(int i=first[x];i;i=next[i]){
			int t=to[i]; if(!w[i] || vis[t]) continue;
			dis[t] = dis[x] + 1; vis[t] = 1;
			if(t==ed) return true;
			q.push(t);
		}
	}return false;
}
int dfs(int u,int flow){
	if(u==ed) return flow;
	int ans=0;
	for(int i=first[u];i;i=next[i]){
		int t=to[i]; 
		if(dis[t] == dis[u] + 1){
			int delta = dfs(t , min(flow , w[i]));
			flow -= delta , ans += delta;
			w[i] -= delta , w[i^1] += delta;
			if(flow==0) break;
		}
	}
	if(flow) dis[u] = -1;
	return ans;
}
int dinic(){
	int ans=0;
	while(bfs()){
		if(int x=dfs(st,inf)) ans+=x;
	}return ans;
}
int main(){
	scanf("%d%d",&m,&n); st=0 , ed=n+1;
	for(int i=1;i<=m;i++) add(st,i,1),add(i,st,0);
	for(int i=m+1;i<=n;i++) add(i,ed,1),add(ed,i,0);
	while(1){
		scanf("%d%d",&x,&y); if(x==-1) break;
		add(x,y,1),add(y,x,0);
	}
	int x=dinic(); if(x==0){printf("No Solution");return 0;}
	printf("%d\n",x); 
	for(int i=2;i<=tot;i+=2)
		if(to[i]!=st && to[i^1]!=st && to[i]!=ed && to[i^1]!=ed && w[i^1])
			printf("%d %d\n",to[i^1],to[i]);
	return 0;
}