codevs1232

最大流dinic打法（这里用了当前弧优化）

diniv当前弧优化讲解

#include<cstdio>
#include<cstring>
#include<cctype>
#include<queue>
#include<algorithm>
#define inf 0x3ffffff
#define maxn 505
using namespace std;
int s,t,head[maxn],cur[maxn],cnt,dep[maxn];
struct data{int to,w,nex;}e[maxn<<2];
inline void read(int &x){
    char ch=getchar();x=0;int f=1;
    while(!isdigit(ch)){if(ch=='-')f=-1;ch=getchar();}
    while(isdigit(ch)){x=(x<<1)+(x<<3)+ch-'0';ch=getchar();}
    x*=f;
}
inline void addedge(int u,int v,int w){
    e[cnt].to=v;e[cnt].w=w;e[cnt].nex=head[u];head[u]=cnt++;
    e[cnt].to=u;e[cnt].w=0;e[cnt].nex=head[v];head[v]=cnt++;
}
inline int bfs(){
    queue<int>q;
    memset(dep,0,sizeof dep);
    dep[s]=1;
    q.push(s);
    while(!q.empty()){
        int u=q.front();q.pop();
        for(int i=head[u];~i;i=e[i].nex){
            int v=e[i].to;
            if(e[i].w>0 && dep[v]==0){
                dep[v]=dep[u]+1;
                if(v==t)return 1;
                q.push(v);
            }
        }
    }
    return dep[t]!=0;
}
inline int dfs(int u,int flow){
    if(u==t)return flow;
    int ans=0,x=0;
    for(int &i=cur[u];~i;i=e[i].nex){
        int v=e[i].to;
        if(e[i].w>0 && dep[v]==dep[u]+1){
            x=dfs(v,min(flow-ans,e[i].w));
            e[i].w-=x;e[i^1].w+=x;
            ans+=x;
            if(ans==flow)return flow;
        }
    }
    if(ans==0)dep[u]=0;
    return ans;
}
inline int dinic(int n){
    int ans=0;
    while(bfs()){
        for(int i=0;i<=n;i++)cur[i]=head[i];ans+=dfs(s,inf);
    }
    return ans;
}
int main(){
    int n,m;read(m);read(n);
    memset(head,-1,sizeof head);
    s=0,t=n+m+1;
    for(int i=1;i<=m;i++)addedge(s,i,1);
    for(int i=m+1;i<=n+m;i++)addedge(i,t,1);
    int u,v;
    while(1){read(u);read(v);if(u==-1 && v==-1)break;addedge(u,v,1);}
    int ans=dinic(t);
    printf("%d\n",ans);
    if(ans==0)return printf("No Solution!\n"),0;
    for(int i=m+1;i<=n+m;i++)
       for(int j=head[i];~j;j=e[j].nex)if(e[j].to!=t && e[j].w==1)printf("%d %d\n",e[j].to,i);
    return 0;
}