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include

include

include

include

include

include

define N 100010

define M 300010

define inf 1000000000

using namespace std;
namespace dinic{
struct node{
int nxt,to,w;
}road[N<<2];
int head[N],cnt=1;
void add(int u,int v,int w)
{
road[++cnt]=(node){head[u],v,w};
head[u]=cnt;
road[++cnt]=(node){head[v],u,0};
head[v]=cnt;
}
int dep[N],cur[N];
queueq;
bool bfs(int s,int t)
{
memset(dep,127,sizeof(dep));
memcpy(cur,head,sizeof(cur));
dep[s]=1;
q.push(s);
while(!q.empty())
{
int u=q.front();
q.pop();
for(int i=head[u];i;i=road[i].nxt)
{
int v=road[i].to;
if(dep[v]>=inf && road[i].w)
{
dep[v]=dep[u]+1;
q.push(v);
}
}
}
return dep[t]<=inf;
}
int dfs(int u,int t,int flow)
{
if(u==t || !flow) return flow;
int ans=0;
for(int i=head[u];i;i=road[i].nxt)
{
int v=road[i].to,f=0;
if(dep[v]==dep[u]+1 && (f=dfs(v,t,min(flow,road[i].w))))
{
flow-=f;
ans+=f;
road[i].w-=f;
road[i^1].w+=f;
if(!flow) break;
}
}
return ans;
}
int dinic(int s,int t)
{
int ans=0;
while(bfs(s,t))
ans+=dfs(s,t,inf);
return ans;
}
}
int col[N];
namespace tarjan{
int nxt[M],to[M],head[N],cnt;
int dfn[N],low[N],idn;
int sta[N],top;
int id[N],tot;
bool in[N];
void add(int u,int v)
{
nxt[++cnt]=head[u];
to[cnt]=v;
head[u]=cnt;
}
void tarjan(int u)
{
dfn[u]=low[u]=++idn;
sta[++top]=u;
in[u]=true;
for(int i=head[u];i;i=nxt[i])
{
int v=to[i];
if(!dfn[v])
{
tarjan(v);
low[u]=min(low[u],low[v]);
}
else if(in[v]) low[u]=min(low[u],dfn[v]);
}
if(dfn[u]==low[u])
{
++tot;
int x;
do{
x=sta[top--];
in[x]=0;
id[x]=tot;
}while(u!=x);
}
}
#define P pair<int,int>
#define MP make_pair
vector

ans;
void work(int s,int t,int n)
{
for(int u=1;u<=t;u++)
for(int i=dinic::head[u];i;i=dinic::road[i].nxt)
if(dinic::road[i].w)
add(u,dinic::road[i].to);
for(int u=1;u<=t;u++)
if(!dfn[u]) tarjan(u);
for(int u=1;u<=n;u++)
if(!col[u])
{
for(int i=dinic::head[u];i;i=dinic::road[i].nxt)
{
int v=dinic::road[i].to;
if(dinic::road[i].w) continue;
if(id[u]!=id[v] && v!=s) ans.push_back(MP(min(u,v),max(u,v)));
}
}
sort(ans.begin(),ans.end());
printf("%d\n",ans.size());
for(int i=0;i<ans.size();i++) printf("%d %d\n",ans[i].first,ans[i].second);
}
}
int nxt[M],to[M],head[N],cnt;
void add(int u,int v)
{
nxt[++cnt]=head[u];
to[cnt]=v;
head[u]=cnt;
}
void dfs(int u,int c)
{
col[u]=c;
for(int i=head[u];i;i=nxt[i])
{
int v=to[i];
if(col[v]==-1) dfs(v,c^1);
}
}
int main()
{
int n,m;
scanf("%d%d",&n,&m);
int s=n+1,t=s+1;
for(int i=1;i<=n;i++) col[i]=-1;
for(int i=1;i<=m;i++)
{
int u,v;
scanf("%d%d",&u,&v);
add(u,v);
add(v,u);
}
for(int i=1;i<=n;i++)
if(col[i]==-1) dfs(i,1);
for(int i=1;i<=n;i++)
if(!col[i]) dinic::add(s,i,1);
else dinic::add(i,t,1);
for(int u=1;u<=n;u++)
if(!col[u])
{
for(int i=head[u];i;i=nxt[i])
{
int v=to[i];
dinic::add(u,v,1);
}
}
dinic::dinic(s,t);
tarjan::work(s,t,n);
return 0;
}

[HAOI2017]新型城市化

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原文地址：https://www.cnblogs.com/Flying2018/p/12237089.html