hdu 3046 最小割

每个栅栏其实就是一条边，修一些栅栏，使得狼不能抓到羊，其实就是求一个割，使得羊全在S中，狼全在T中。

 #include <cstdio>
 #include <cstring>
 #include <vector>
 #include <queue>
 #define maxn 40010
 #define oo 0x3f3f3f3f
 #define clr(arr,n) memset(&arr,0,sizeof(arr[0])*(n+1))
 using namespace std;

 struct Edge {
     int u, v, f;
     Edge( int u, int v, int f ):u(u),v(v),f(f){}
 };
 struct Dinic {
     int n, src, dst;
     vector<Edge> edge;
     vector<int> g[maxn];
     int dep[maxn], cur[maxn];

     void init( int n, int src, int dst ) {
         this->n = n;
         this->src = src;
         this->dst = dst;
         for( int u=; u<=n; u++ )
             g[u].clear();
         edge.clear();
     }
     void add_edge( int u, int v, int f ) {
         g[u].push_back( edge.size() );
         edge.push_back( Edge(u,v,f) );
         g[v].push_back( edge.size() );
         edge.push_back( Edge(v,u,) );
     }
     bool bfs() {
         queue<int> qu;
         clr( dep, n );
         qu.push(src);
         dep[src] = ;
         while( !qu.empty() ) {
             int u=qu.front();
             qu.pop();
             for( int t=; t<g[u].size(); t++ ) {
                 Edge &e=edge[g[u][t]];
                 if( e.f && !dep[e.v] ) {
                     dep[e.v] = dep[e.u]+;
                     qu.push( e.v );
                 }
             }
         }
         return dep[dst];
     }
     int dfs( int u, int a ) {
         if( u==dst || a== ) return a;
         int remain=a, past=, na;
         for( int &t=cur[u]; t<g[u].size(); t++ ) {
             Edge &e = edge[g[u][t]];
             Edge &ve = edge[g[u][t]^];
             if( e.f && dep[e.v]==dep[e.u]+ && (na=dfs(e.v,min(e.f,remain))) ) {
                 remain -= na;
                 past += na;
                 e.f -= na;
                 ve.f += na;
                 if( remain== ) break;
             }
         }
         return past;
     }
     int maxflow() {
         int flow = ;
         while(bfs()) {
             clr( cur, n );
             flow += dfs(src,oo);
         }
         return flow;
     }
 };

 int n, m;
 int idx[][], id_clock;
 int map[][];
 int dx[] = { +,  };
 int dy[] = { , + };
 Dinic D;

 int main() {
     for( int cas=; ; cas++ ) {
         if( scanf( "%d%d", &n, &m )!= ) return ;
         id_clock = ;
         for( int i=; i<=n; i++ )
             for( int j=; j<=m; j++ ) {
                 scanf( "%d", &map[i][j] );
                 idx[i][j] = ++id_clock;
             }
         D.init( id_clock+, id_clock+, id_clock+ );
         for( int i=; i<=n; i++ )
             for( int j=; j<=m; j++ )
                 for( int d=; d<; d++ ) {
                     int ni = i+dx[d];
                     int nj = j+dy[d];
                     if( <=ni&&ni<=n && <=nj&&nj<=m ) {
                         int u = idx[i][j];
                         int v = idx[ni][nj];
                         D.add_edge( u, v,  );
                         D.add_edge( v, u,  );
                     }
                 }
         for( int i=; i<=n; i++ )
             for( int j=; j<=m; j++ ) {
                 if( map[i][j]== )
                     D.add_edge( D.src, idx[i][j], oo );
                 if( map[i][j]== )
                     D.add_edge( idx[i][j], D.dst, oo );
             }
         printf( "Case %d:\n%d\n", cas, D.maxflow() );
     }
 }