UVA 820 Internet Bandwidth 因特网宽带（无向图，最大流，常规）

题意：给一个无向图，每条边上都有容量的限制，要求求出给定起点和终点的最大流。

思路：每条无向边就得拆成2条，每条还得有反向边，所以共4条。源点汇点已经给出，所以不用建了。直接在图上跑最大流就可以了。

 #include <bits/stdc++.h>
 #define LL long long
 #define pii pair<int,int>
 #define INF 0x7f7f7f7f
 using namespace std;
 const int N=;
 const int mod=1e9+;
 int s, t;

 int path[N], flow[N];
 vector<int> vect[N];

 struct node
 {
     int from, to, cap, flow;
     node(){};
     node(int from,int to,int cap,int flow):from(from),to(to),cap(cap),flow(flow){};
 }edge[];
 int edge_cnt;

 void add_node(int from,int to,int cap,int flow)
 {
     edge[edge_cnt]=node(from,to,cap,flow);
     vect[from].push_back(edge_cnt++);
 }

 int BFS(int s,int e)
 {
     deque<int> que(,s);
     flow[s]=INF;
     while(!que.empty())
     {
         int x=que.front();
         que.pop_front();
         for(int i=; i<vect[x].size(); i++)
         {
             node e=edge[vect[x][i]];
             if(!flow[e.to] && e.cap>e.flow)
             {
                 flow[e.to]=min(flow[e.from],e.cap-e.flow);
                 path[e.to]=vect[x][i];
                 que.push_back(e.to);
             }
         }
         if(flow[e]) return flow[e];
     }
     return flow[e];
 }

 int max_flow(int s,int e)
 {
     int ans_flow=;
     while(true)
     {
         memset(path,,sizeof(path));
         memset(flow,,sizeof(flow));

         int tmp=BFS(s,e);
         if(!tmp)    return ans_flow;
         ans_flow+=tmp;

         int ed=e;
         while(ed!=s)
         {
             int t=path[ed];
             edge[t].flow+=tmp;
             edge[t^].flow-=tmp;
             ed=edge[t].from;
         }
     }
 }
 int main()
 {
     freopen("input.txt", "r", stdin);
     int n, a, b, v, c, j=;
     while(scanf("%d",&n),n)
     {
         edge_cnt=;
         memset(edge,,sizeof(edge));
         for(int i=; i<=n+; i++)   vect[i].clear();

         scanf("%d%d%d", &s, &t, &c);
         for(int i=; i<c; i++)
         {
             scanf("%d%d%d",&a,&b,&v);
             add_node(a, b, v, );
             add_node(b, a, , );
             add_node(b, a, v, );
             add_node(a, b, , );
         }
         printf("Network %d\n",++j);
         printf("The bandwidth is %d.\n\n", max_flow(s ,t) );
     }
     return ;
 }

AC代码