Drainage Ditches

 1 #include<iostream>
 2 #include<cstring>
 3 #include<algorithm>
 4 #include<cstdio>
 5 #include<queue>
 6 #define INF 100000000
 7 using namespace std;
 8 const int N=405;
 9 const int M=1000000;
10 struct Node{
11    int v;
12    int f;
13    int next;
14 }edge[M];
15 int n,m,u,v,w,cnt,sx,ex;
16 int head[N],pre[N];
17 void init(){
18     cnt=0;
19     memset(head,-1,sizeof(head));
20 }
21 void add(int u,int v,int w){
22     edge[cnt].v=v;
23     edge[cnt].f=w;
24     edge[cnt].next=head[u];
25     head[u]=cnt++;
26     edge[cnt].f=0;
27     edge[cnt].v=u;
28     edge[cnt].next=head[v];
29     head[v]=cnt++;
30 }
31 bool BFS(){
32   memset(pre,0,sizeof(pre));
33   pre[sx]=1;
34   queue<int>Q;
35   Q.push(sx);
36  while(!Q.empty()){
37      int d=Q.front();
38      Q.pop();
39      for(int i=head[d];i!=-1;i=edge[i].next    ){
40         if(edge[i].f&&!pre[edge[i].v]){
41             pre[edge[i].v]=pre[d]+1;
42             Q.push(edge[i].v);
43         }
44      }
45   }
46  return pre[ex]>0;
47 }
48 int dinic(int flow,int ps){
49     int f=flow;
50      if(ps==ex)return f;
51      for(int i=head[ps];i!=-1;i=edge[i].next){
52         if(edge[i].f&&pre[edge[i].v]==pre[ps]+1){
53             int a=edge[i].f;
54             int t=dinic(min(a,flow),edge[i].v);
55               edge[i].f-=t;
56               edge[i^1].f+=t;
57             flow-=t;
58              if(flow<=0)break;
59         }
60 
61      }
62       if(f-flow<=0)pre[ps]=-1;
63       return f-flow;
64 }
65 int solve(){
66     int sum=0;
67     while(BFS())
68         sum+=dinic(INF,sx);
69     return sum;
70 }
71 int main() {
72     while(~scanf("%d%d",&n,&m)) {
73          init();
74         for(int i=1;i<=n; i++) {
75            scanf("%d%d%d",&u,&v,&w);
76            add(u,v,w);
77         }
78        sx=1;ex=m;
79       printf("%d\n",solve());
80     }
81     return 0;
82 }