POJ2135 Farm Tour 最小费用流

　　很容易看出来时最小费用流，但这里要注意是无向边，所以要建立两条边。为了满足退流时，花费还是最小，反向边的花费要为相反数。
 1 //STATUS:G++_AC_32MS_980KB
 2 #include<stdio.h>
 3 #include<stdlib.h>
 4 #include<string.h>
 5 #include<math.h>
 6 #include<iostream>
 7 #include<string>
 8 #include<algorithm>
 9 #include<vector>
10 #include<queue>
11 #include<stack>
12 #include<map>
13 using namespace std;
14 #define LL __int64
15 #define pii pair<int,int>
16 #define Max(a,b) ((a)>(b)?(a):(b))
17 #define Min(a,b) ((a)<(b)?(a):(b))
18 #define mem(a,b) memset(a,b,sizeof(a))
19 #define lson l,mid,rt<<1
20 #define rson mid+1,r,rt<<1|1
21 const int MAX=1010,INF=0x3f3f3f3f;
22 const LL LLNF=0x3f3f3f3f3f3f3f3fLL;
23 
24 struct Edge{
25     int u,v,cap,w;
26 }e[MAX*40];
27 
28 int d[MAX],first[MAX],next[MAX*40],inq[MAX],p[MAX];
29 int n,m,s,t,mt;
30 
31 void adde(int a,int b,int c,int val){
32     e[mt].u=a,e[mt].v=b,e[mt].cap=c,e[mt].w=val;
33     next[mt]=first[a],first[a]=mt++;
34     e[mt].u=b,e[mt].v=a,e[mt].cap=0,e[mt].w=-val;
35     next[mt]=first[b],first[b]=mt++;
36 }
37 
38 int Mincost()
39 {
40     int i,j,x,a,cost=0;
41     queue<int> q;
42     p[s]=-1;
43     while(1){
44         a=INF;
45         mem(d,0x3f);
46         mem(inq,0);
47         d[s]=0;
48         q.push(s);
49         while(!q.empty()){
50             x=q.front();q.pop();
51             inq[x]=0;
52             for(i=first[x];i!=-1;i=next[i]){
53                 if(e[i].cap && d[e[i].u]+e[i].w<d[e[i].v]){
54                     d[e[i].v]=d[e[i].u]+e[i].w;
55                     p[e[i].v]=i;
56                     if(!inq[e[i].v]){
57                         q.push(e[i].v);
58                         inq[e[i].v]=1;
59                     }
60                 }
61             }
62         }
63         if(d[t]==INF)break;
64         for(i=p[t];i!=-1;i=p[e[i].u])
65             if(e[i].cap<a)a=e[i].cap;
66         for(i=p[t];i!=-1;i=p[e[i].u]){
67             e[i].cap-=a;
68             e[i^1].cap+=a;
69         }
70         cost+=d[t]*a;
71     }
72     return cost;
73 }
74 
75 int main()
76 {
77  //   freopen("in.txt","r",stdin);
78     int i,j,a,b,c;
79     while(~scanf("%d%d",&n,&m))
80     {
81         t=n;
82         s=mt=0;
83         mem(first,-1);
84         for(i=0;i<m;i++){
85             scanf("%d%d%d",&a,&b,&c);
86             adde(a,b,1,c);
87             adde(b,a,1,c);
88         }
89         adde(s,1,2,0);
90 
91         printf("%d\n",Mincost());
92     }
93     return 0;
94 }