POJ2135 Farm Tour

 1 /*by SilverN*/
 2 #include<iostream>
 3 #include<algorithm>
 4 #include<cstring>
 5 #include<cstdio>
 6 #include<cmath>
 7 #include<queue>
 8 using namespace std;
 9 const int INF=1000000;
10 const int mxn=2100;
11 int head[mxn],dis[mxn],pr[mxn];
12 bool inqu[mxn];
13 int n,m;
14 int s,t;
15 int ans;
16 int cnt=1;
17 struct edge{
18     int from,to,next,v,c;
19 }e[mxn*30];
20 void add_edge(int f,int t,int v,int c){
21     e[++cnt]=(edge){f,t,head[f],v,c};head[f]=cnt;
22     e[++cnt]=(edge){t,f,head[t],0,-c};head[t]=cnt;
23 }
24 bool SPFA(){
25     queue<int>q;
26     memset(inqu,false,sizeof(inqu));
27     for(int i=0;i<=t;i++)dis[i]=INF;
28     dis[s]=0;
29     inqu[s]=1;
30     q.push(s);
31     int i,j;
32     while(!q.empty()){
33         int u=q.front();q.pop();
34         inqu[u]=false;
35         for(i=head[u];i;i=e[i].next){
36             int v=e[i].to;
37             if(e[i].v && dis[u]+e[i].c<dis[v]){
38                 dis[v]=dis[u]+e[i].c;
39                 pr[v]=i;
40                 if(!inqu[v]){
41                     q.push(v);
42                     inqu[v]=true;
43                 }
44             }
45         }
46     }
47     return dis[t]!=INF;
48 }
49 void mcf(){
50     int i;
51     while(SPFA()){
52         int temp=INF;
53         for(i=pr[t];i;i=e[pr[i]].from)temp=min(temp,e[i].v);
54         ans+=dis[t]*temp;
55         for(i=pr[t];i;i=e[pr[i]].from){
56             e[i].v-=temp;
57             e[i^1].v+=temp;
58         }
59     }
60 }
61 int main(){
62     scanf("%d%d",&n,&m);
63     s=0;t=n+1;
64     int x,y,c;
65     for(int i=1;i<=m;i++){
66         scanf("%d%d%d",&x,&y,&c);
67         add_edge(x,y,1,c);
68         add_edge(y,x,1,c);
69     }
70     add_edge(s,1,2,0);
71     add_edge(n,t,2,0);
72     mcf();
73     printf("%d\n",ans);
74 }