UVA-10480 Sabotage

题目分析：网络流求最小割的模板题，难点在于求出最小割集。求出最小割集的方法是在求网络流之后跑一遍BFS对能够跑到的点进行标记（能跑到表示该点与起点处于同一割点集），然后遍历边，则最小割边即于起点被标记，终点没被标记的边，去重依次输出即可。

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<set>
#include<vector>
#include<iomanip>
#include<queue>
#include<map>
#include<stack>
using namespace std;
typedef long long LL;
typedef unsigned long long ULL;
const int maxn=550;
const LL INF = 100000000000000;
int n,m;
struct edge
{
LL from,to,cap,flow;
edge(LL f,LL t,LL c,LL fl):from(f),to(t),cap(c),flow(fl){}
};
struct Dinic
{
int n,m,s,t;
vector<edge> edges;
vector<int> G[maxn];
bool vis[maxn];
int d[maxn];
int cur[maxn];
void init(int n,int s,int t,int m)
{
this->n=n,this->s=s,this->t=t,this->m=m;
edges.clear();
for(int i=1;i<=n;i++)
G[i].clear();
}
void addedge(int from,int to,int cap)
{
edges.push_back(edge{from,to,cap,0});
edges.push_back(edge{to,from,0,0});
int m=edges.size();
G[from].push_back(m-2);
G[to].push_back(m-1);
}
bool bfs()
{
memset(vis,0,sizeof(vis));
queue<int> Q;
Q.push(s);
d[s]=0;
vis[s]=1;
while(!Q.empty())
{
int x=Q.front();
Q.pop();
for(int i=0;i<G[x].size();i++)
{
edge& e=edges[G[x][i]];
if(!vis[e.to]&&e.cap>e.flow)
{
vis[e.to]=1;
d[e.to]=d[x]+1;
Q.push(e.to);
}
}
}
return vis[t];
}
LL dfs(LL x,LL a)
{
if(x==t||a==0)
return a;
LL flow=0,f;
for(int &i=cur[x];i<G[x].size();i++)
{
edge& e =edges[G[x][i]];
if(d[x]+1==d[e.to]&&(f=dfs(e.to,min(a,e.cap-e.flow)))>0)
{
e.flow+=f;
edges[G[x][i]^1].flow-=f;
flow+=f;
a-=f;
if(a==0)
break;
}
}
return flow;
}
LL Maxflow()
{
LL flow=0;
while(bfs())
{
memset(cur,0,sizeof(cur));
flow+=dfs(s,INF);
}
return flow;
}
void GetEdge()
{
memset(vis,0,sizeof(vis));
queue<int> Q;
Q.push(s);
vis[s]=1;
while(!Q.empty())
{
int x = Q.front();
Q.pop();
for(int i=0;i<G[x].size();i++)
{
edge& e = edges[G[x][i]];
if(!vis[e.to]&&e.cap>e.flow)
{
vis[e.to]=1;
Q.push(e.to);
}
}
}
int Map[maxn][maxn];
memset(Map,0,sizeof(Map));
for(int i=0;i<edges.size();i++)
{
edge e =edges[i];
if(vis[e.from]&&!vis[e.to]&&!Map[e.from][e.to])
{
Map[e.from][e.to]=1;
printf("%lld %lld\n",e.from,e.to);
}
}
printf("\n");
}
}DC;
int main()
{
while(scanf("%d%d",&n,&m)!=EOF)
{

if(n==0&&m==0)
break;
DC.init(n,1,2,m);
for(int i=0;i<m;i++)
{

4000
LL a,b,c;
scanf("%lld%lld%lld",&a,&b,&c);
DC.addedge(a,b,c);
DC.addedge(b,a,c);
}
DC.Maxflow();
DC.GetEdge();
}

return 0;
}