[POJ 2135] Farm Tour

 1 #include <stdio.h>
 2 #include <string.h> 
 3 #include <queue>
 4 using namespace std;
 5 const int B=50010;
 6 int tot, head[B], next[B], flow[B], w[B], to[B], d[B], S, T, pre[B], bi[B], oq[B];
 7 bool vis[B];
 8 int n,m;
 9 queue<int> q;
10 inline void add(int u,int v,int _w,int _flow) {
11     to[tot]=v; flow[tot]=_flow; w[tot]=_w; next[tot]=head[u]; head[u]=tot++;
12     to[tot]=u; flow[tot]=0; w[tot]=-_w; next[tot]=head[v]; head[v]=tot++;    
13 }
14 inline int spfa() {
15     memset(d,0x3f3f3f3f,sizeof(d));
16     memset(oq,0,sizeof(oq));
17     memset(vis,0,sizeof(vis));
18     memset(pre,-1,sizeof(pre));
19     memset(bi,0,sizeof(bi));
20     while(!q.empty()) q.pop();
21     d[S]=0;
22     vis[S]=1;
23     pre[S]=S;
24     q.push(S);
25     while(!q.empty()) {
26         int top=q.front();
27         q.pop();
28         vis[top]=0;
29         oq[top]++;
30         if(oq[top]>n+2) return -1;
31         for (int i=head[top];i!=-1;i=next[i]) 
32             if(flow[i]&&d[top]+w[i]<d[to[i]]) {
33                 d[to[i]]=d[top]+w[i];
34                 if(!vis[to[i]]) {
35                     vis[to[i]]=1;
36                     q.push(to[i]);
37                 }
38                 pre[to[i]]=top;
39                 bi[to[i]]=i;
40             }
41     }
42     if(d[T]==0x3f3f3f3f) return -1;
43     else return d[T];
44 }
45 inline int mincostflow(int S,int T) {
46     int flowx, cost;
47     flowx=cost=0;
48     while(1) {
49         int minf=0x3f3f3f3f;
50         int Spfa = spfa();
51         if (Spfa==-1||Spfa==0x3f3f3f3f) break;
52         for (int i=T;i!=S;i=pre[i])
53             if (flow[bi[i]]<minf) minf=flow[bi[i]];
54         flowx += minf;
55         cost += Spfa*minf;
56         for (int i=T;i!=S;i=pre[i]) {
57             flow[bi[i]]-=minf;
58             flow[bi[i]^1]+=minf;
59         }
60         //printf("--%d %d\n",flowx,cost);
61     }
62     return cost;
63 }
64 
65 int main() {
66     memset(head,-1,sizeof(head));
67     scanf("%d%d",&n,&m);
68     for (int i=1;i<=m;++i) {
69         int _u,_v,_w;
70         scanf("%d%d%d",&_u,&_v,&_w);
71         add(_u,_v,_w,1);
72         add(_v,_u,_w,1);
73     }
74     // 添加源和汇 
75     S=0, T=n+1;
76     add(S,1,0,2);
77     add(n,T,0,2);
78     printf("%d\n",mincostflow(S,T)); 
79     return 0;
80 }