题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=4289
思路：求最小花费，最小割应用，将点权转化为边权，拆点，(i,i+n)之间连边，容量为在城市i的花费，然后就是若u,v之间有路，则连边（u+n,v),(v+n,u).最后就是跑最大流了。
1 #include<iostream> 2 #include<cstdio> 3 #include<cstring> 4 #include<algorithm> 5 #include<queue> 6 using namespace std; 7 #define MAXN 444 8 #define MAXM 44444444 9 #define inf 1<<30 10 11 struct Edge{ 12 int v,cap,next; 13 }edge[MAXM]; 14 15 int n,m,vs,vt,NE,NV; 16 int head[MAXN]; 17 18 void Insert(int u,int v,int cap) 19 { 20 edge[NE].v=v; 21 edge[NE].cap=cap; 22 edge[NE].next=head[u]; 23 head[u]=NE++; 24 25 edge[NE].v=u; 26 edge[NE].cap=0; 27 edge[NE].next=head[v]; 28 head[v]=NE++; 29 } 30 31 int level[MAXN],gap[MAXN]; 32 void bfs(int vt) 33 { 34 memset(level,-1,sizeof(level)); 35 memset(gap,0,sizeof(gap)); 36 level[vt]=0; 37 gap[level[vt]]++; 38 queue<int>que; 39 que.push(vt); 40 while(!que.empty()){ 41 int u=que.front(); 42 que.pop(); 43 for(int i=head[u];i!=-1;i=edge[i].next){ 44 int v=edge[i].v; 45 if(level[v]!=-1)continue; 46 level[v]=level[u]+1; 47 gap[level[v]]++; 48 que.push(v); 49 } 50 } 51 } 52 53 int pre[MAXN],cur[MAXN]; 54 int SAP(int vs,int vt) 55 { 56 bfs(vt); 57 memset(pre,-1,sizeof(pre)); 58 memcpy(cur,head,sizeof(head)); 59 int maxflow=0,aug=inf; 60 int u=pre[vs]=vs; 61 gap[0]=NV; 62 while(level[vs]<NV){ 63 bool flag=false; 64 for(int &i=cur[u];i!=-1;i=edge[i].next){ 65 int v=edge[i].v; 66 if(edge[i].cap>0&&level[u]==level[v]+1){ 67 flag=true; 68 pre[v]=u; 69 u=v; 70 aug=min(aug,edge[i].cap); 71 if(v==vt){ 72 maxflow+=aug; 73 for(u=pre[v];v!=vs;v=u,u=pre[u]){ 74 edge[cur[u]].cap-=aug; 75 edge[cur[u]^1].cap+=aug; 76 } 77 aug=inf; 78 } 79 break; 80 } 81 } 82 if(flag)continue; 83 int minlevel=NV; 84 for(int i=head[u];i!=-1;i=edge[i].next){ 85 int v=edge[i].v; 86 if(edge[i].cap>0&&level[v]<minlevel){ 87 cur[u]=i; 88 minlevel=level[v]; 89 } 90 } 91 if(--gap[level[u]]==0)break; 92 level[u]=minlevel+1; 93 gap[level[u]]++; 94 u=pre[u]; 95 } 96 return maxflow; 97 } 98 99 int main() 100 { 101 int u,v,w; 102 while(~scanf("%d%d",&n,&m)){ 103 NE=0,NV=2*n; 104 memset(head,-1,sizeof(head)); 105 scanf("%d%d",&vs,&vt); 106 for(int i=1;i<=n;i++){ 107 scanf("%d",&w); 108 Insert(i,i+n,w); 109 } 110 while(m--){ 111 scanf("%d%d",&u,&v); 112 Insert(u+n,v,inf); 113 Insert(v+n,u,inf); 114 } 115 printf("%d ",SAP(vs,vt+n)); 116 } 117 return 0; 118 }