题目地址

易错点：

  • 需要用pre[]数组记录方案.
#include<cstdio>
#include<iostream>
#include<queue>
#include<cstring>
using namespace std;
const int MAXN=1000010,MAXM=1000010;
int n,s,t;
struct Edge{
    int from,to,w,nxt;
}e[MAXM];
int head[MAXN],edgeCnt=1;
void addEdge(int u,int v,int w){
    e[++edgeCnt].from=u;
    e[edgeCnt].to=v;
    e[edgeCnt].w=w;
    e[edgeCnt].nxt=head[u];
    head[u]=edgeCnt;
}
int d[MAXN];
bool bfs(){
    memset(d,0,sizeof(d));
    queue<int> q;
    d[s]=1;q.push(s);
    while(!q.empty()){
        int nowV=q.front();q.pop();
        for(int i=head[nowV];i;i=e[i].nxt){
            int nowNode=e[i].to;
            if(!d[nowNode]&&e[i].w){
                d[nowNode]=d[nowV]+1;
                if(nowNode==t)return 1;
                q.push(nowNode);
            }
        }
    }
    return d[t]!=0;
}
int pre[MAXN];
int Dinic(int x,int flow){
    if(x==t)return flow;
    int rest=flow;
    for(int i=head[x];i&&rest;i=e[i].nxt){
        int nowV=e[i].to;
        if(d[nowV]==d[x]+1&&e[i].w){
            int k=Dinic(nowV,min(rest,e[i].w));
            if(!k)d[nowV]=0;
            if(k)pre[nowV]=x;
            e[i].w-=k,e[i^1].w+=k;
            rest-=k;
        }
    }
    return flow-rest;
}
const int INF=2e9;
int main(){
    int m;
    scanf("%d%d",&m,&n);
    s=n+1,t=n+2;
    int u,v;
    while(1){
        scanf("%d%d",&u,&v);
        if(u==-1){
            break;
        }
        addEdge(u,v,INF);
        addEdge(v,u,0);
    }
    for(int i=1;i<=m;i++){
        addEdge(s,i,1);
        addEdge(i,s,0);
    }
    for(int i=m+1;i<=n;i++){
        addEdge(i,t,1);
        addEdge(t,i,0);
    }
    int ans=0;
    while(bfs()){
        ans+=Dinic(s,2e9);
    }
    if(!ans){
        cout<<"No Solution!"<<endl;
        return 0;
    }
    cout<<ans<<endl;
    for(int i=m+1;i<=n;i++){
        if(pre[i])
            printf("%d %d\n",i,pre[i]);
    }
    return 0;
}