poj3204Ikki's Story I - Road Reconstruction（最大流求割边）

链接

最大流=最小割  这题是求割边集 dinic求出残余网络 两边dfs分别以源点d找到可达点 再以汇点进行d找到可达汇点的点

如果u,v为割边 那么s->u可达 v->t可达 并且为饱和边

 #include <iostream>
 #include <cstdio>
 #include <cstring>
 #include <queue>
 #include <algorithm>
 using namespace std;
 const int INF = ;
 const int N = ;
 #define M 100005
 struct node
 {
     int u,v,next;
     int w;
 }edge[M],p[M];
 int head[N],t,vis[N],pp[N],dis[N];
 int st,en;
 bool vis1[N],vis2[N];
 void init()
 {
     t=;
     memset(head,-,sizeof(head));
 }
 void add(int u,int v,int w)
 {
     edge[t].u = u;
     edge[t].v = v;
     edge[t].w = w;
     edge[t].next = head[u];
     head[u] = t++;
     edge[t].u = v;
     edge[t].v = u;
     edge[t].w = ;
     edge[t].next = head[v];
     head[v] = t++;
 }
 int bfs()
 {
     int i,u;
     int w;
     memset(dis,,sizeof(dis));
     queue<int>q;
     q.push(st);
     dis[st]=  ;
     while(!q.empty())
     {
         u = q.front();
         q.pop();
         for(i = head[u] ; i != - ; i = edge[i].next)
         {
             int v = edge[i].v;
             w = edge[i].w;
             if(!dis[v]&&w>)
             {
                 dis[v] = dis[u]+;
                 q.push(v);
             }
         }
     }
     if(dis[en]>) return ;
     return ;
 }
 int dfs(int u,int te)
 {
     int i;
     int s;
     if(u==en) return te;
     int tmp = te;
     for(i = head[u] ; i != - ; i = edge[i].next)
     {
         int v = edge[i].v;
         int w = edge[i].w;
         if(w>&&dis[v]==dis[u]+&&(s=dfs(v,min(te,w))))
         {
             edge[i].w-=s;
             edge[i^].w+=s;
             tmp-=s;
         }
     }
     return te-tmp;
 }
 int dinic()
 {
     int flow = ;
     while(bfs())
     {
         flow+=dfs(st,INF);
     }
     return flow;
 }
 void dfs1(int u)
 {
     int i;
     for(i = head[u] ; i != - ; i= edge[i].next)
     if(!vis1[edge[i].v]&&edge[i].w>)
     {
         vis1[edge[i].v] = ;
         dfs1(edge[i].v);
     }
 }
 void dfs2(int u)
 {
     int i;
     for(i = head[u] ; i != - ; i= edge[i].next)
     {
         int v =  edge[i].v;
         //cout<<v<<" "<<edge[i].w<<endl;
         if(!vis2[edge[i].v]&&edge[i^].w>)
         {
         vis2[edge[i].v] = ;
         dfs2(edge[i].v);
         }
     }
 }
 int main()
 {
     int n,m,i;
     while(scanf("%d%d",&n,&m)!=EOF)
     {
         init();
         memset(vis1,,sizeof(vis1));
         memset(vis2,,sizeof(vis2));
         for(i =  ;i <= m; i++)
         {
             int u,v,c;
             scanf("%d%d%d",&u,&v,&c);
             u++,v++;
             add(u,v,c);
         }
         st = ,en = n;
         int kk = dinic();
         //cout<<kk<<endl;
         vis1[st] = ;
         dfs1(st);
         vis2[en] = ;
         dfs2(en);
         int ans = ;
         dinic();
         for(i = ; i < t; i+=)
         {
             int u = edge[i].u;
             int v = edge[i].v;
             int w = edge[i].w;
             //cout<<u<<" "<<v<<" "<<vis1[u]<<" "<<vis2[v]<<endl;
             if(vis1[u]&&vis2[v]&&w<=)
             ans++;
         }
         cout<<ans<<endl;
     }
     return ;
 }