hdu - 1532 Drainage Ditches (最大流)

  1 // Rujia Liu
  2 // 因为图较大，所以采用Dinic而不是EdmondsKarp
  3 // 得益于接口一致性，读者无须理解Dinic就能使用它。
  4 #include<cstdio>
  5 #include<cstring>
  6 #include<queue>
  7 #include<algorithm>
  8 using namespace std;
  9 
 10 const int maxn = 50*50+10;
 11 
 12 const int INF = 1000000000;
 13 
 14 struct Edge
 15 {
 16   int from, to, cap, flow;
 17 };
 18 
 19 bool operator < (const Edge& a, const Edge& b)
 20 {
 21   return a.from < b.from || (a.from == b.from && a.to < b.to);
 22 }
 23 
 24 struct Dinic
 25 {
 26     int n, m, s, t;
 27     vector<Edge> edges;    // 边数的两倍
 28     vector<int> G[maxn];   // 邻接表，G[i][j]表示结点i的第j条边在e数组中的序号
 29     bool vis[maxn];        // BFS使用
 30     int d[maxn];           // 从起点到i的距离
 31     int cur[maxn];         // 当前弧指针
 32 
 33     void init(int n)
 34     {
 35         for(int i = 0; i < n; i++) G[i].clear();
 36         edges.clear();
 37     }
 38 
 39     void AddEdge(int from, int to, int cap)
 40     {
 41         edges.push_back((Edge){from, to, cap, 0});
 42         edges.push_back((Edge){to, from, 0, 0});
 43         m = edges.size();
 44         G[from].push_back(m-2);
 45         G[to].push_back(m-1);
 46     }
 47 
 48     bool BFS()
 49     {
 50         memset(vis, 0, sizeof(vis));
 51         queue<int> Q;
 52         Q.push(s);
 53         vis[s] = 1;
 54         d[s] = 0;
 55         while(!Q.empty())
 56         {
 57             int x = Q.front(); Q.pop();
 58             for(int i = 0; i < G[x].size(); i++)
 59             {
 60                 Edge& e = edges[G[x][i]];
 61                 if(!vis[e.to] && e.cap > e.flow)
 62                 {
 63                     vis[e.to] = 1;
 64                     d[e.to] = d[x] + 1;
 65                     Q.push(e.to);
 66                 }
 67             }
 68         }
 69         return vis[t];
 70     }
 71 
 72     int DFS(int x, int a)
 73     {
 74         if(x == t || a == 0) return a;
 75         int flow = 0, f;
 76         for(int& i = cur[x]; i < G[x].size(); i++)
 77         {
 78             Edge& e = edges[G[x][i]];
 79             if(d[x] + 1 == d[e.to] && (f = DFS(e.to, min(a, e.cap-e.flow))) > 0)
 80             {
 81                 e.flow += f;
 82                 edges[G[x][i]^1].flow -= f;
 83                 flow += f;
 84                 a -= f;
 85                 if(a == 0) break;
 86             }
 87         }
 88         return flow;
 89     }
 90 
 91     int Maxflow(int s, int t)
 92     {
 93         this->s = s; this->t = t;
 94         int flow = 0;
 95         while(BFS())
 96         {
 97             memset(cur, 0, sizeof(cur));
 98             flow += DFS(s, INF);
 99         }
100         return flow;
101     }
102 };
103 
104 Dinic g;
105 
106 int main()
107 {
108   //  freopen("a.txt","r",stdin);
109     int n,m,a,b,c;
110     while(~scanf("%d%d",&m,&n))
111     {
112         g.init(n);
113         for(int i=0;i<m;i++)
114         {
115             scanf("%d%d%d",&a,&b,&c);
116             g.AddEdge(a,b,c);
117         }
118         printf("%d\n",g.Maxflow(1, n));
119     }
120     return 0;
121 }