P3376 【模板】网络最大流

 1 #include <cstdio>
 2 
 3 #define MIN(a,b) ( a<b ?a :b)
 4 #define INF (1<<20)
 5 
 6 using namespace std;
 7 
 8 const int N(100000+15);
 9 int n,m,s,t,u,v,w;
10 int head[N<<1],sumedge;
11 struct Edge
12 {
13     int to,next,val;
14     Edge(int to=0,int next=0,int val=0):
15         to(to),next(next),val(val){}
16 }edge[N<<1];
17 
18 void ins(int from,int to,int val)
19 {
20     edge[++sumedge]=Edge(to,head[from],val);
21     head[from]=sumedge;
22 }
23 
24 int h,tail,que[N],deep[N];
25 
26 bool BFS(int t)
27 {
28     for(int i=1;i<=n;i++) deep[i]=-1;
29     deep[s]=0; h=tail=0;que[tail++]=s;
30     for(;h<tail;h++)
31     {
32         int front=que[h]; if(front==t) return true;
33         for(int i=head[front];i;i=edge[i].next)
34         {
35             int go=edge[i].to;
36             if(deep[go]<0&&edge[i].val)
37             {
38                 deep[go]=deep[front]+1;
39                 que[tail++]=go;
40             }
41         }
42     }
43     return false;
44 }
45 
46 int DFS(int now,int flow)
47 {
48     if(now==t||!flow) return flow;
49     int flux=0;
50     for(int i=head[now],go=edge[i].to;i;i=edge[i].next,go=edge[i].to)
51     {
52         if(deep[go]!=deep[now]+1||edge[i].val<=0) continue;
53         int min=DFS( go, MIN(flow,edge[i].val) );
54         flow-=min;
55         flux+=min;
56         edge[i].val-=min;
57         edge[i^1].val+=min;
58         if(!flow) return flux;
59     }
60     if(!flux) deep[now]=-1;
61     return flux;
62 }
63 
64 int Dinic(int s,int t)
65 {
66     int sum=0;
67     for(;BFS(t);)
68         sum+=DFS(s,INF);
69     return sum;
70 }
71 
72 int main()
73 {
74     scanf("%d%d%d%d",&n,&m,&s,&t);
75     for(int i=1;i<=m;i++)
76     {
77         scanf("%d%d%d",&u,&v,&w);
78         ins(u,v,w); ins(v,v,0);
79     }
80     int ans=Dinic(s,t);
81     printf("%d\n",ans);
82     return 0;
83 }