Drainage Ditches(Dinic最大流)

http://poj.org/problem?id=1273

用Dinic求最大流的模板题，注意会有重边。

邻接矩阵建图

 #include<stdio.h>
 #include<string.h>
 #include<queue>
 #include<algorithm>
 using namespace std;
 const int maxn = ;
 const int INF = 0x3f3f3f3f;
 int flow[maxn][maxn];//残量
 int m,n;
 int dis[maxn];
 int bfs()//按层数“建”图，就是对每层的点用dis标记它到源点的层数
 {
     queue<int>que;
     memset(dis,-,sizeof(dis));
     while(!que.empty())
         que.pop();
     dis[] = ;
     que.push();

     while(!que.empty())
     {
         int u = que.front();
         que.pop();

         for(int i = ; i <= n; i++)
         {
             if(flow[u][i] >  && dis[i] < )
             {
                 dis[i] = dis[u]+;
                 que.push(i);
             }
         }
     }

     if(dis[n] > )
         return ;
     else return ;
 }
 //dfs寻找路径上的最小流量
 int dfs(int s, int mf)
 {
     int a;
     if(s == n)
         return mf;
     for(int i = ; i <= n; i++)
     {
         if(dis[i] == dis[s] +  && flow[s][i] >  && (a = dfs(i,min(mf,flow[s][i]))))
         {
             flow[s][i] -= a;
             flow[i][s] += a;
             return a;
         }
     }
     return ;
 }

 int main()
 {
     while(~scanf("%d %d",&m,&n))
     {
         int u,v,w;
         memset(flow,,sizeof(flow));
         for(int i = ; i < m; i++)
         {
             scanf("%d %d %d",&u,&v,&w);
             flow[u][v] += w;
         }
         int ans = ,res;
         while(bfs())//bfs寻找源点到汇点是否有路
         {
             res = dfs(,INF);
             if(res > )
                 ans += res;
         }
         printf("%d\n",ans);
     }
     return ;

 }

邻接表建图

 #include<stdio.h>
 #include<string.h>
 #include<queue>
 #include<algorithm>
 using namespace std;
 const int maxn = ;
 const int INF = 0x3f3f3f3f;
 int m,n,cnt;
 struct node
 {
     int u,v,w;
     int next;
 }edge[maxn];
 int p[maxn];
 int dis[maxn];
 void add(int u, int v, int w)
 {

     edge[cnt].u = u;
     edge[cnt].v = v;
     edge[cnt].w = w;
     edge[cnt].next = p[u];
     p[u] = cnt++;

     edge[cnt].u = v;
     edge[cnt].v = u;
     edge[cnt].w = ;
     edge[cnt].next = p[v];
     p[v] = cnt++;
 }

 int bfs()
 {
     queue<int> que;
     while(!que.empty())
         que.pop();
     memset(dis,-,sizeof(dis));
     dis[] = ;
     que.push();

     while(!que.empty())
     {
         int u = que.front();
         que.pop();

         for(int i = p[u]; i != -; i = edge[i].next)
         {
             if(edge[i].w >  && dis[ edge[i].v ] < )
             {
                 dis[ edge[i].v ] = dis[u] + ;
                 que.push(edge[i].v);
             }
         }
     }
     if(dis[n] > )
         return ;
     else return ;
 }

 int dfs(int s, int mf)
 {
     if(s == n)
         return mf;
     int a;
     int tf = ;
     for(int i = p[s]; i != -; i = edge[i].next)
     {
         int v = edge[i].v;
         int w = edge[i].w;
         if(dis[v] == dis[s] +  && w >  && (a = dfs(v,min(mf,w))))
         {
             edge[i].w -= a;
             edge[i^].w += a;
             return a;
         }
     }
     if(!tf)//若找不到小流，从这个点到不了汇点，把这个点从分层图删去
         dis[s] = -;
     return tf;
 }

 int main()
 {
     while(~scanf("%d %d",&m,&n))
     {
         int u,v,w;
         cnt = ;
         memset(p,-,sizeof(p));
         for(int i = ; i < m; i++)
         {
             scanf("%d %d %d",&u,&v,&w);
             add(u,v,w);
         }
         int ans = ;
         int res;
         while(bfs())
         {
             res = dfs(,INF);
             if(res > )
                 ans += res;
         }
         printf("%d\n",ans);
     }

     return ;
 }